Any opportunity cost is that cost which you give up in order to pursue a different thing. In this problem, you give up your $11,000 job to go to school. However you are going to spend $2300 (tuition is $2000 and books are $300) to go to school.
In short: If you took the job, you would make $11,000. If you go to summer school, you spend $2,300. However, you are in school, and both spending and not making the money. Your opportunity cost is their sum - $13,300, because you gave up the job and are spending money on books and tuition.
Notice that the living expenses are not mentioned - that is a fixed cost and that change is not included.
The opportunity cost is $13,300.
