Let's solve the question, is very easy!
The answer for f(3) is -9 when the function is f(x)=-4x+3.
Now lets do it when f(X) = (1/7)(7^x) if f(3)
The answer is f(3)=49 when f(X) = (1/7)(7^x).
Remember only replace the number in every x in the function.