So I'm doing find the coefficient of x^4 in (3x-(1/x^2))^22

I've done where a = 3x and b = 1/x^2

(22)

(K) (3x)^K (-1/x^2)^(22-K)

(22)

(K) (3^K) (x^K) (-1)^(22-K) x^-2(22-K) ----- I think this is where I've stuffed up

(22)

(K) (3^K) (-1)^(22-K).... not sure from here lol.

Please help! Would be appreciated.