Got it now.
We are told that the triangle is equilateral (all sides are congruent). One side of the equilateral triangle is on the square. So this means the sides of the square are congruent to the sides of the triangle.
Look at the markings I have made on the sides of the triangle and square, indicating which triangles formed are isosceles. It should be pretty apparent if you know anything about isosceles triangles.