I saw the image that should have accompanied this question.
Δ DMU & Δ MPA both are right triangles.
DMU: short leg = 12 ; hypotenuse = 15
MPA: short leg = 0.5x + 6 ; hypotenuse = 2y - 3
x = 12 ; y = 9
12 = 0.5(12) + 6
12 = 6 + 6
12 = 12
15 = 2y - 3
15 = 2(9) - 3
15 = 18 - 3
15 = 15
SAS postulate states that if 2 sides and 1 angle of 1 triangle is congruent to the 2 sides and 1 angle of another triangle. Then, these triangles are congruent.
Since sides are already computed, you need to look for the congruent angles of each triangle. In this case, since both are right triangles, its congruent angle is 90°