iii)
If the roots are distinct then it passes through x axis 3 times. Therefore f(u) must be a negative value and f(v) must be a positive value. A positive value times a negative value is alwyas lesser than 0.
therefore f(u).f(v)<0
f'(x) = 3x^2 +b = 0
therefore x= +(-b/3)^1/2 and...