Find critical numbers calculus

I need some help with steps :
Find all critical numbers for f(x) = 2 times sqrt (9-x^2)
Thanks

f = 2*(9 - x^2)^.5

f = 2*u^.5
u = 9-x^2
du = -2x dx

df = u^(-.5) du
df = (9-x^2)^(-.5) * -2x dx

df/dx = (9-x^2)^(-.5) * -2x

find all x such that df/dx = 0

x = sqrt(9), -sqrt(9), 0