Lawnboy18,
It looks very simple, Using your rider, I would make one pass on the bottom of the hill, then one pass at the top, then mow what was left with the small mower, shouldn't be more than two passes.
The flat part at the bottom of the hill will guide your mower on the slope, in other words, you can't slide down the hill if you are at the bottom already.
The top of the hill is level so you won't have any problem there, so that leaves you with what's left and that isn't worth worrying about.
That tiny bit isn't worth getting another mower.