No clue how to do this

8L - 2 = P

Step-by-step explanation:

Please, just one question per post.  I'll focus on #15

As a reminder, the formula for the perimeter of a rectangle is

P = 2L + 2W, where L is the length and W the width.

Next, we represent the width in terms of the length:  W = 3L - 1.

We want the perimeter of this rectangle in terms of the length, L, with the width, W, not appearing in the final result.

Here P = 2L + 2W = 2L + 2(3L - 1), or

P as a function of L = 2L + 6L - 2 = 8L - 2 = P