Assume f(x)=-2+8 and g(x)=3x what is the value of (f+)

The meaning of (f + g)(-1) is you substitute -1 to x of whatever polynomial you obtain after adding the polynomials f(x) and g(x).
                    f(x) + g(x) = -2x + 8 + 3x = x + 8
Substitute -1,
                        (f + g)(-1) = -1 + 8 = 7.
Thus, the answer is 7.

Yes, two vertical lines can be complementary