Looking at the largest right triangle, we have the hypotenuse being 15 so we know the side opposite to the 30 degrees will be 15/2 (side z) because its a 30-60-90 triangle. The smaller right triangle to the left with z as its hypotenuse has y opposite the 60 degree triangle. We know due to 30-60-90 triangles that it will have to be 15/4 * sqrt(3) because the hypotenuse is 2x and the side opposite the 60 degree angles is xsqrt(3).
So the blank has 15 in the numerator, 4 in the denominator and 3 under the square root.
