#### Answer

The side is 9 feet, and the altitude is 4 feet.

#### Work Step by Step

We are told:
$ l = 2h +1 $
Using the formula for the area of a triangle, we find:
$ A = .5lh \\ 18 = .5(2h+1)(h) \\ 0 = h^2 + .5h - 18 \\ 0 = 2h^2 + h -36 \\ (h-4)(2h+9)$
Since h can't be negative, we see that h=4. This means that: $ l = 2(4) + 1 = 9. $