Okay, so I'm doing a TON of identities in trig, and have come to a point where I'm working on problems such as:

cos of 30 is

I've forgotten what it's called but I need a step by step guide to simplifying this! (it simplifies down to an exact answer of

Edit: Yes, I've done college algebra (and aced it) but I haven't had to simplify problems like this in about 6 or 7 months, so I've lost the skill!