They're essentially asking you for the antiderivative. You could say, well, 2x is the derivative of x squared. You could also say 2x is the derivative of x squared plus 1. You could also say that 2x is the derivative of x squared plus pi. I think you get the general idea. If you wanted to write it in the most general sense, you would write that 2x is the derivative of x squared plus some constant. This is what you would consider the antiderivative of 2x.