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

The perimeter of a parallelogram is given by P = 2(a + b).
Given P = 18, AD = x, and AB = x+5.
So, 18 = 2(x + x+5) => 9 = 2x + 5 => 2x = 4 => x = 2.