When provided algebra for Quantitative Comparison problems, consider solving algebraically first, because the approach is likely to be more efficient than plugging in.

In this case we have:

Quantity A	Quantity B
f(3, 4) + g(3, 4)	f(4, 3) + g(4, 3)

Now, in the root statement f(x, y) = 2x + y and g(x + y) = x + 2y | Since both quantities sum these equations, stack and sum the full equations to efficiently compare the quantities.

2x + y
+ x + 2y
3x + 3y

Without plugging in, it can be determined that when adding the two functions of both quantities the x and y coefficients will remain equal. Therefore, since the x and y values are the same in both quantities, the quantities themselves will always be equal, which is choice C.