15. Find x. The Ab and CD must be of equal length in a rectangle. so we can also say that 4x-60 is equal to 30-x
4x-60 = 30-x | + x (same oparation on both sides)
5x - 60 = 30 | +60 (as above. to have x on one side and, seperated, pure numbers on the other side)
5x = 90 | divide by 5 on both sides
x = 18
