My problem is regarding this function:

and Im suppose to write the Laplace transformation of this product using the theorem:

I understand the principle behind this problem, and I know the answer is * L(cos(3t) in terms of t-pi/6). Its just I can't figure out how to rewrite in terms of . I tried to do cos use the addition property, and end up getting stuck with a that I dont know what to do with. Can someone help me out?