GMAT Quantitative Reasoning Practice Test 3

The easiest way is to back solve the question.
Take answer $\mathrm{\$}10,000$, if that is the amount Harris invested in the video mat; the annual return was $\mathrm{\$}1,200$. Therefore, there are $\mathrm{\$}35,000$ left to invest in the car-cleaning machine, $8\%$ of 35,000 is $\mathrm{\$}2,800$. Sum them up; the total return is like the question asked $-\mathrm{\$}4,000$.