You recognize that, of the numbers in Y, 20 is not a square. You also recognize that the remaining values of Y correspond to the squares of the values of X:
To solve the equation for x, you have to isolate the x variable on one side of the equation. To do this, do the opposite of any operation applied to x to remove other variables from the x's side of the equation:
x/c = d
multiply both sides of the equation by c to isolate x:
Triangle STU is congruent to triangle UTX is the missing step. AAS (angle-angle-side) is a method of proving 2 triangles congruent, and using the already proved information, you can find the triangles that are congruent by AAS.