Let's group the cubic function: 
.
The first two roots are
however to find the last two roots we need to solve the square equation:
. Now we know the discriminator, using the discriminator we're able to find the roots of the equation: 
. The roots of the square equation are 
and 
.
The roots of the cubic function are, 
and 
.
The first two roots are
The roots of the cubic function are