Hello,
If you can write the joint pdf as the product of a function which only depends on x, and a function which only depends on y, then they're independent.
You have been told how to do this. You are expected to have sufficient understanding of things so that explanations such as those given make sense.
Go back to your textbook or class notes and review this material. Also, click on 5.2 here: Probability density function - Wikipedia, the free encyclopedia