I'm not quite sure if I'm answering the question...?
"Use the double argument property, to express cos 60° in terms of sin 30°."
My response:
The next question asks the exact same thing, except in reverse:
"Use the double argument property: to express cos 60° in terms of sin 30°."
Would I just take my work and swap everything over the equal sign? Assuming I approached this problem correctly...