2 angles are complementary if the sum of their measures is 90°.
For example if m(P) = 41° and m(Q)=49°, then P and Q are complementary.
Thus A and B are complementary means that m(A)=m(B)=90°:
(3x+5°) + (2x-15°) =90°
5x-10°=90°
5x=100°
x=20°
Thus
m(A)=3x+5°=3* 20°+5°=60°+5°=65°
m(B)=2x-15°=2*20°-15°=40°-15°=25°