Calculus: Find y' and y'' where y = ln(3x) / x^3 .........HELP

Calculus: Find y' and y''. y = ln(3x) / x^3

Don't know what I'm doing wrong. I keep getting for y' =(1-3ln3x)/x^4 and for y"= (7-12ln3x)/x^5 , but thats wrong. HELP

Using product rule,
y' = (1/x)(1/x^3) - 3ln(3x)/x^4 = (1-3ln3x)/x^4
y'' = -(3/x)(1/x^4) - 4(1-3ln3x)/x^5 = -(7-12ln3x)/x^5

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Attn: You missed "-" sign.