3) ABCD - parallelogram. AB = x+5, AD = x, P = 18. Find x.

The perimeter of a parallelogram is given by P = 2(a+b). In this case, P = 18, a = x+5, and b = x. So, 18 = 2((x+5) + x). Simplifying, 9 = 2x + 5. Subtracting 5 from both sides gives 4 = 2x. Dividing by 2 yields x = 2.