Help with a geometry word problem! Triangle Side lengths! Suppose a chair has a seat that's an isosceles triangle and the congruent sides measure...

If you have stated the question properly, then all you
have to remember is that congruent triangles have both sides and angles that are
congruent.  This is as opposed to similar triangles that have congruent angles, but not
necessarily sides.

If these two triangles are congruent,
they both have the same side lengths and the same total perimeter.   Given that, we can
see that the sides of the second triangle (and the first) are 1.5 feet, 1.5 feet and 2.1
feet.

The reason for this is that the perimeter of a
triangle is the sum of all its sides.  We know that two of the sides are 1.5 feet each. 
Those two sides, then, come out to 3.0 feet in length.  If the perimeter of the whole
triangle is 5.1 feet, then the remaining side must be 2.1 feet.