Hello, ZKG9744!
If you "just started trig", this is a very fancy problem.
We have: .
Multiply by
We have: .
If you think of a right angled triangle with the side opposite a having length equal to 4 and the side adjacent to a having the length 9. The hypotenuse, by pythagoras' theorem is
Then with the basic definition of sin: . Similar thing with cos.
You might find it easier to do the problem by first expanding