Okay, they tell us that f(x) is 3, so lets just bring that into f(x)=x^2 -2x
We can solve by guess and check, but we can also use the quadratic formula.
Let's move 3 to the other side.
x^2 -2x -3=0
We get 3 and (-1), using the quadratic formula. If you don't know what that is, you can find how to use it on google. I hope you find this useful.