So the board walk adds x feet to either side the pond and board walk form a rectangle with width 30+ 2x and length 40+ 2x feet.

A rectangle of length 30+ 2x feet and width 40+ 2x feet has area (30+ 2x)(40+ 2x)= 2000.The combined surface area of the pond and the boardwalk is 2000 ft^2. What is the width of the boardwalk?

Yes, that is the surface area of the boardwalk (I believe you mean 2000 ft^2 rather than "2000^2")- you could then calculate the lengths of the four "sides" on the rectangle but the above is easier.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?

You could, for example, "dissect" the boardwalk as two rectangles of length 40 and width x and two rectangles of length 30+ x (draw a picture to see why) and width x so area 2(x)(40)+ 2(x)(30+ x)= 800. That give you the same equation as before.The answer is 5ft.