Question

Suppose f(x) = 23 + x and g(x) = x - 9 /3
Find the value of (g - f)(18).

Answer 1

that would become (g-f)(18)=(18-9/3)-(23+18)
all you do is replace the x with the value

Answer 2

Okay thanks, I got the answer correct. How do we know that 18 is x?

Answer 3

well if you look at (g-f)(18), this is the same thing as g(18)-f(18).
g(x)=x-9/3
g(18)=18-9/3
f(x)=23+x
f(18)=23+18
does that help?