One property of a rhombus is that the diagonals are perpendicular. This means that the diagonals intersect at 90 degree angles (right angles).
So we know that angle JNK is 90 degrees
angle JNK = 90 degrees
5x - 15 = 90
5x - 15 + 15 = 90+15
5x = 105
5x/5 = 105/5
x = 21
Answer: x = 21