An isosceles triangle is a triangle with two sides with the same measure. The third side does not give a specific measurement so you can't assume all sides are congruent. This means you can't assume the bottom side is congruent just because two sides are of the same length, that piece could be decimals shorter or larger and it still wouldn't be the same value. Your assumption would be correct if instead of showing the side measurements it showed two angles with a given value of 60 degrees. This is because you can assume the final angle must be 60 degrees as a triangles angles add up to 180 degrees.