Find all values of x (if any) for which f may have a relative maximum or a relative minimum?

1. Let f(x)= 7x^3 - 4x^2
Find all values of x (if any) for which f may have a relative maximum or a relative minimum.

2. Let f(x)= x - (6/x^2)
Find all values of x (if any) for which f may have a relative maximum.

1. Let f(x)= 7x^3 - 4x^2

f '(x) = 21x^2 - 8x = 0
x(21x - 8) = 0
x = 0 or x = 8/21

2. Let f(x)= x - (6/x^2)

f'(x) = 1 + 12/x^3 = (x^3 + 12)/x^3
f'(x) = 0 if x^3 + 12 = 0

x = - ³√12