complex numbers - Find the roots of the polynomial? (Cardano's Method)

$y^3-\frac7{12}y-\frac7{216}$

This is part of Cardano's method, so I've gotten my first root to be:

$y_1=\sqrt[3]{\frac7{432}+i\sqrt{\frac{49}{6912}}}+\sqrt[3]{\frac7{432}-i\sqrt{\frac{49}{6912}}}$

I am at a loss on how to evaluate this. I know this is one of three real roots, but I don't know what to do now.