Convert harmonic equation to polar form

[ssb]

Homework Statement

Convert
cos(7 t) + sin(7 t)
into polar form of
A cos(\omega_0 t - \delta)

This is a review problem where once we convert this to polar form we are to give amplitude, period and delay (shift). I can answer this question if someone can point me to a website tutorial that tells the process of converting this problem. I really don't expect someone to answer this for me because I need to learn how to do it on my own.

The Attempt at a Solution

I have not made an attempt as of yet, however i know there is a trig identity
cos(x)cos(y) + sin(x)sin(y) = cos(x-y)

I was given this advice BUT I don't know what to do because:
cos(7)cos(t) doesn't equal cos(7t) ... right?

 

[rock.freak667]

Let Acos(\theta-\delta)=cos(7t)+sin(7t) then just equate and solve