You can make this process simpler by replacing the variables given with ones that are easier to solve for. For instance:
f = 5^x + 2x
g = 3x - 6
Find f + g.
The question is asking you to add the equations, so add them as you would usually add polynomials:
5^x + 2x + 3x - 6
Seeing as only the middle two expressions are similar, those are the only ones that can be simplified:
5^x + 5x - 6
The correct choice is A.