equation\

I think you answered your own question ...with your question......did you type something incorrectly?

I agree with EP....however, let's suppose, just for the sake of argument, that 

f(x)  =  x^2 + 3x  - 2     and we want to find f ( x - 3)

So we have   (x - 3)^2 + 3(x - 3)  - 2     simplify

x^2 - 6x + 9  + 3x - 9  - 2

x^2 - 3x - 2  

Look at the graphs, here :   https://www.desmos.com/calculator/2fwmyzx9zo

Note that  f(x -3)  shifts f(x) to the right by 3 units  [which we should expect ]