Okay so I'm guessing that they are two different formulas the f(x)= 3x and the f(x)= -5x-2. If that's is true then you would just have to plug in -2 to the x's. So then it would be f(x)= 3(-2) which if you solve it would give you f(x)= -6 and for the second one it would be f(x)= -5(-2)-2 so then you would solve for it and get f(x)= 8.
