relative extrema?

Question

use the second derivative to find the relative extrema of
f(x) = x - 4 * square root of (x+1)

Answer 1

f(x) = x - 4√(x+1)
f'(x) = 1 - 2/√(x+1)
f'(x) = (x+1)^-(3/2)
√(x+1) = 2
x + 1 = 4
x = 3
f(3) = 3 - 4*2 = - 5
f''(3) = √8, so
(3, - 5) is a minimum