You need to set a variable to represent the width, let's say "w'.
width = w
length (l) = 20 + w
Area = length * width
A = (20 + w) * w
See how I substituted the length in terms of w, above? We also know the area, so plug that in:
A = (20 + w) * w
1500 = (20 + w) *w
Solve for w. If it's easier, rewrite as:
1500 = w(20 + w)
Distribute the right side, and solve for w (width).
Then plug back into the equation for length --> length = 20 + w to find the other dimension
