A student's on a bathroom scale that's at rest in an elevator. The scale reads 836N. As the elevator moves up, the scale reading increases to 935N then decreases back to 836N. Find the acceleration of the elevator.
To answer this question do I find the difference between the two weights and then use that difference as my Fnet and with that solve for mass. Then with this mass plug it into the a = Fnet/m equation?

Yes - that's it. The extra "weight" felt by the scale is due to the acceleration of the student's mass. So the delta in force of 935N - 836N is due to the student's mass (which you can find from 836N=mg) accelerating at a = deltaF/m.