P= 8w+12. Perimeter of a rectangle is equal to twice the length plus twice the width, or P= 2l +2w. If the length is 3 times the width plus 6, the length can be written as 3w+6. P=2l + 2w so plug in 3w+6 for the length to get P=2(3w+6) + 2w. Simplify and P= 8w+12.