Solving an equation with trigonometrics...

Hello,

I need to solve the following equation in order to find the value of "y"
using Maxima:

x=-1/2*arctan(y/2)+3*%pi/8;

I would like to get a value of "y" instead of this:
[arctan(y/2)=(3*%pi-8*x)/4]

Can anyone help please?

Thanks in advance,

Julien.