Firstly this question relates to an assignment so please do not provide answer.

I am asked to find the roots of a function which appears to be a cubic function.

Function

$\displaystyle \frac{x^3}{3} - x^2 - 3x$

I am given a graph which clearly shows 3 roots, which are approx -2, 0 and 5. However I need to work out the exact points.

I have read the

The "Cubic Formula" which talks about the cubic formula however I honestly cannot get my head around it. Does anyone know of a better link which explains it better or can explain it better ?

Unfortunately the book I have been given doesn't touch on this formula.