The formula for the perimiter of a rectangle is p = 2l + 2w. The question is asking us to solve for l in this formula. In order to do that, we do the following:

p = 2l + 2w *Subtract 2w to the other side to isolate l*

p - 2w = 2l *Divide by 2 to solve for l*

**l = (p - 2w)/2**

Now we have to use this equation to solve for l if p = 12 cm and w = 2.5 cm

l = (p - 2w)/2 *Let's plug in our values first*

l = (12 - (2 * 2.5))/2 *Let's multiply 2 by 2.5 to get 5*

l = (12 - 5)/2 *Now subtract 12 by 5 to get 7*

l = 7/2 *Now divide 7 by 2 to get 3.5*

**l = 3.5 cm**