Question 1:
F(x) and g(x) are like variables, just plug into the equation.
f(x) + g(x) = (x + 6) + (12x - 7)
x+6+12x-7 = 13x-1
Question 2: f(3) + g(-1)
You plug in the x-values into the equation, and then take the answer and add them together.
f(3) = 3+4
g(-1) = 12(-1)-6
f(3) = 7
g(-1) = -18
7 + (-18) = -11
Question 3:
This is similar to question 1, plug in the variables and simplify.
9x - (7x+3)
Remember to distribute the "-"
9x - 7x - 3
2x - 3