Normal force usually is -mg, but in this case it will be -mg+the force applied on it by the accelerating window. The right hand side is the y-components of the frictional force and normal force.
For the x-direction, my equation would be Fnet=ma, with Fnet being the forces in the x-direction...
I think I am on the right path but I'm having trouble cancelling out the mass of the wallet. I drew a free body diagram and know that there are three forces acting on the wallet: gravity, friction, and normal force (including the van's acceleration).
The y-component of the friction and normal...