Let's denote the number of office buildings as \( x \).
According to the problem, the window cleaner can clean 21 windows per office building and 42 windows for a school. The total number of windows they can clean is 126.
The equation that represents the total number of windows cleaned is:
\[ 21x + 42 = 126 \]
So the correct response is:
21x + 42 = 126.