A boardwalk that is x feet wide is built around a rectangular pond. The pond is 30 ft wide and 40 ft long. The combined surface area of the pond and the boardwalk is 2000 ft^2. What is the width of the boardwalk?

I tried 2000^2 - (30*40) = 800ft^2 as the surface area of the boardwalk but I don't know if I was on the right track. How should I do this question?

The answer is 5ft.