Prove that

$\displaystyle tan 4 theta = 4 tan theta (1-tan^2 theta)/1-6tan^2 theta + tan ^4 theta$

(Sorry about that. I don't know how to use LaTEX, it's all messed up. I used the MATH tag and it showed an error and so I used the TEX tag).

I don't know where to start. What trig identities should I use?