- I think it's possibly this "The figure must be a square because the diagonals of a square bisect the right angles." but there's also "The figure must be a rhombus because it has exactly 2 pairs of congruent angles." buuut since the first one seems more better i'm assuming then that's probably the right answer.
see the attached figure to better understand the problem
we know that
The equilateral triangle has three equal sides
so
in the equilateral triangle ABC

the height of the triangle is the segment BD
in the right triangle BCD
Applying the Pythagorean Theorem

solve for BD

substitute the values



therefore
<u>the answer is</u>
the height of the triangle is 
The answer is 15, to find this take 225 and square root
The area of a square is length times length( or side times side) . 15 times 15 is 225