Rewrite the product as a sum or difference.

20cos (36t) cos (6t)

The applicable identity is ...

... cos (a) cos (b) = (1/2) (cos (a+b) + cos (a-b))

Let a=36t, b=6t and substitute this into your expression. Then you have ...

... 20cos (36t) cos (6t) = 20· (1/2) (cos (36t+6t) + cos (36t-6t))

... = 10 (cos (42t) + cos (30t))

... = 10cos (42t) + 10cos (30t)