A rectangular garden is 20 ft longer than it is wide. its area is 800 ft^2. what are its dimensions?

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