Hey spyder12.
Try calculating the derivative first.
I actually solved the problem---
f(x)= 4x - 5x^(1/3)
derivative of f(x)= 4 - (1.66667 / x^(2/3))
x= -.268957 , .268957
f(x)= 4(-.3) - 5(-.3)^(1/3) = +
f(x)= 4(.1) - 5(.1)^(1/3) = -
f(x)= 4(.3) - 5(.3)^(1/3) = +
Increasing - (-inf, -.268957) U (.268957, inf)
Decreasing - (-.268957, .268957)