help w/ derivative quotient rule

I tried using the quotient rule for this problem and got:

(-7x^(-7/2) + 14x - 12x^(3/2) ) 4x

This didn't match up with my friend or mathway's answer and I'm not quite sure where I went wrong. 

I found f'(x) to be: (-6x^(-7/2) + 21x - 16x^(3/2))

and g'(x) to be: (x^-1/2)

Then, I just followed the quotient rule and combined like terms.

Thanks for any help!