find f'(a) for f(x)=1/sqrt(13-7x)

Question

i got my answer to be 7 / (7(13-7x)sqrt(13-7x))
but it says i am wrong

please help

Answer 1

u = 13-7x
u' = -7

y = u^-1/2
y' = -1/2 * u^-3/2 * u'

= 7/[2 * (13-7x)^3/2]