f (x − 3), if f (x) = x2

$$\\f(x)=x^2\\ so\\ f(x-3)=(x-3)^2=x^2-6x+9$$

Did you mean f(x) - 3?

if you mean f(x) = (x - 3) if x = 2, then you just plug in 2 for x

f(2) = (2-3)

f(2) = -1