How would you support an incoming granite slab here?

I'm trying to make this into a desk workstation with a single slab of granite. The red pony walls are framed with 2x4 and covered with Sheetrock. My granite guy is coming out to install but I have to figure out the best way to support this roughly 90" span which is 34" deep at the deepest.

It's going to be a desk for a computer with lots of legroom below.

How would you best support the granite slab?