Answer: Augmenting his income.
Explanation: Extra work means having an official fixed job and, in addition, after working hours, or in your free time (such as weekends), you dedicate yourself to other activities that can also generate income. Unlike what some people think, extra work does not need to be something completely different from what you usually do. Of course you can choose to formalize your hobby and earn money with it, but these tasks can also happen in other conventional work.