D. 18
Step-by-step explanation:
∠DAC ≅ ∠BCA and the diagonals cross at right angles, so the two marked angles are complementary.
(5x -18) +x = 90
6x = 108 . . . . . . . . . simplify, add 18
x = 18 . . . . . . . . . . . divide by 6