I am given a critical bare slab reactor 150cm thick (a0=150cm) consisting of a homogenous mixture of U235 and graphite. I am stuck on the part which asks me to calculate the critical atom dennsity. I have solved for the mass of the fuel in terms of the mass of the moderator. I would solve for the mass of the moderator by multiplying the volume of the reactor by the density of graphite and then plug that into the relation I have found for the mass of the moderator and the mass of the fuel, however since a slab reactor is defined only in width, I'm not sure how to find the volume of the reactor. Thanks for any help.