Do you see that when we change
f(x) = 7x - 5
to
f(2) = 7(2) - 5
we are simply saying to substitute in 2 for x? Well that is what we are doing:
f(2) = 7(2) - 5
f(2) = 14 - 5
f(2) = 9
Now we find g(4) the same way:
g(x) = 4(x)
g(4) = 4(4)
g(4) = 16
Now we find the difference of the two functions:
f(2) - g(4)
9 - 16
-7
So, the answer is B.