If your amount (4) is greater than 0, then you would use the equation -2x - 5.

So, for an official way to show your work:

f(4) = -2x - 5

f(4) = -2(4) - 5

f(4) = -8 - 5

f(4) = -13

4 doesn't equal -13, neither is -13 greater than 0, therefore this equation is inaccurate.

I believe that your answer has no solution due to it being a false equation. Therefore, the amount of solutions that your equation has is none. Feel free to correct me if I'm wrong though, since it has been a while since I have reviewed this..!