Final answer:
We put the -15x and the +2x into the boxes and not the +1x and the -30x because the -15x and the +2x are like terms, while the +1x and the -30x are not.
Step-by-step explanation:
The reason we put the -15x and the +2x into the boxes and not the +1x and the -30x is because we are simplifying an algebraic expression. In this case, we are combining like terms. Like terms are those that have the same variable raised to the same exponent. So, when we have -15x and +2x, both have the variable x raised to the power 1 and therefore they are like terms. On the other hand, the +1x and the -30x have different coefficients (1 and -30) and cannot be combined as they have different values. Therefore, we only put the like terms into the boxes to simplify the expression.