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)
