Use the formula for lowering the powers to simplify the expression:
(a) If $\cos^2 3 x - \sin^2 3 x = \cos (f(x))$, then
$f(x) =$ ;
(b) If $\cos^4 3 x - \sin^4 3 x = \cos (g(x))$, then
$g(x) =$ .
Hint: For part (b), use $a^4-b^4=(a^2-b^2)(a^2+b^2)$!