How do i find the derivative of

Question

f(t)= 20- 30/rooot 9 + t^2

heres how i did it

f'(x)= 20' - 30/root(9+t^2) '
= 0 - 30/root(9+t^2) '
(Quotient rule)
f'(x)= 30' (root(9+t^2) - 30(root(9+t^2) '

f'(x)= -30(0.5(9+t^2)^-0.5) (2t)

the answer at the back of the book is 30t/(9+t^2)^1/2.... but i don't knohow to get it.