In Macbeth, what do Malcolm and Donalbain decide at the end of Act 2 Scene 3? Quote lines to support the response.

The slain body of King Duncan has just been discovered by
Macduff and the alarm was sounded. All the nobles are now gathered in the courtyard,
including Malcolm and Donalbain.

Malcolm, the eldest and
proclaimed heir to the throne whispers to Donalbain:

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Why do we hold our
tongues,

That most may claim this argument for
ours?

He is asking his
brother why they remain silent and allow others to speak on their behalf. It is a matter
that affects them most directly and they should therefore speak up. Donalbain, in his
reply, states:

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What should be spoken here,
where our
fate,
Hid in an auger-hole, may rush, and seize us?
Let 's
away;
Our tears are not yet
brew'd.

He says that they
should speak about their fate (their possible deaths) which could be hiding anywhere in
Macbeth's castle. Donalbain expresses fear that they might be killed next, for whoever
planned their father's murder would definitely also want to be rid of them, since they
would be a hurdle to whatever ambitions the murderer might aspire. He recommends that
they should flee, since they have not even had an opportunity to feel sorrowful at their
tragic loss.

Malcolm states that their immediate flight is
more important than expressing their sorrow. The urgency of their situation cannot be
ignored and they have to depart post-haste, they might be murdered if they
tarry.

Malcolm requests that he and his brother not keep
company with the rest of the men and he states that expressing sorrow without real
feeling is a duty that some do without any true conviction or depth, it is merely an act
which they commit to out of courtesy. He obviously does not want to show grief just
because it is expected of him. He wants to express genuine sorrow. For a dishonest man
it would be easy to do so. He tells Donalbain that he will head for
England.

Donalbain declares that he will flee to Ireland.
It is clear that he believes that going their separate ways will ensure their safety,
for it would make it difficult for their enemies to find them at once. Donalbain clearly
implies that he suspects Macbeth when he says that 

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There's daggers in men's smiles:
the near in blood,
The nearer
bloody.

Macbeth
is their closest relative and he stands to benefit most if they should be out of the
way. The crown would pass to him. Malcolm uses a very apt metaphor by stating that the
murderous scheme has not yet reached finality. He compares it to an arrow still flying
towards its target. They are that target and they therefore need to steal away quickly,
leaving everyone unaware that they have departed. He
says:

there's
warrant in that theft
Which steals itself, when there's no mercy
left.

He means that there is
a greater chance of success if they should leave secretly and not make a fuss about
their departure.

It is unfortunate that their actions
create suspicion that they were responsible for their father's assassination and
ironically allows Macbeth to claim the throne.