Answer:
D
Explanation:
The diagonals of a rhombus bisect the angles, thus
∠ ABC = ∠ CBD = 5x - 18 and
∠ BAC = ∠ DAC = x
The diagonals are perpendicular bisectors of each other, thus
the angles round the intersection of the diagonals = 90°
Consider the triangle on the left of the rhombus, then
5x - 18 + x + 90 = 180 ( sum of angles in a triangle )
6x + 72 = 180 ( subtract 72 from both sides )
6x = 108 ( divide both sides by 6 )
x = 18 → D