How do I work out energy stored in a capacitor bank?
Since W=0.5*C*V^2, you just have to find out the total capacitance of the three capacitors combined and apply the formula with the given Vb = 80V.

I have calculated a total capacitance of 30uF which gives me  a stored energy of 96mJ.

So, have a look if you calculated the equivalent capacitance correctly.