When you see f(x)=[equation] and then f(number). Simply remember that you just have to use the number given and replace the x in the equation with it. Though the given “number” may sometimes be another equation that you replace the x with.
So here we are given the equation f(x)=-x+1 and asked to find f(-10). Simply replace x with -10.
f(x)=-x+1
f(x)=10+1 (two negatives equal positive)
f(x)=11