Since F (x) =3x + 4 and you are given that x =2, you would then substitute x for 2. This would lead the problem to look like f (2) = 3(2) + 4. You then use PEMDAS (parentheses, exponents, multiplication, division, addition, subtraction) so you would multiply 3 and 2 leaving you with f(x) = 6 + 4. After this you then would add 6 and 4 leaving you with f(x) = 10. This would mean your final answer would be f(x) = 10! I hope this helps!
