A.
To evaluate f(8), we substitute x with 8 in our expression.
Answer: f(8) = 50
B.
To evaluate f(x+6), we do the same but substitute x with x+6 instead.
Expand the parenthesis by multiplying 6 with x and 6.
Answer: f(x+6) = 6x + 38
C.
Again, we're doing the same thing, just substituting x with -x.
Answer: f(-x) = -6x+2