We are given an equation where two sets of two equal parts make up a whole value, and one set is twice the value of the other. In other words:
l = 2w
2l + 2w = 84
This isn't yet possible to solve in this form, but we can make it really easy to solve by removing one of the variables. We know l = 2w, so if we express l in terms of w, we only have the variable and constant on opposing sides of an equation.
2l + 2w = 84
4w + 2w = 84
6w = 84
6w/6 = 84/6
w = 14
So how do we find the length? Easy. Return back to the length to width equation.
l = 2w
l = 2(14)
l = 28
The length of the rectangle is 28 feet.