My only options are the ones I provided in relevant equations. I assume that WT includes the weight of the box in the truck too so I don't think that's the issue here. Here's the sketch, the two overlapping vectors at the bottom are WT and Nb.
Homework Statement
A large box containing your new computer sits on the bed of your pickup truck. You are stopped at a red light. The light turns green and you stomp on the gas and the truck accelerates. To your horror, the box starts to slide toward the back of the truck. (Assume that the...