First lets consider the regular numbers here. 15 + 24. That gives us 39. Next we consider the variables. There's 0 a's in the first part, and 1 in the second part. So you simply get 1a added on top of the 39. Next, the x variable. There's one on each part, so just add them together to get x^2 (Note that you must do it this way rather than getting 2x since you're only adding x's, so when adding/subtracting variables on its own, its expressed as an exponent). With all this in mind, you'd get answer A