Divide 100 by any number under +100 to get decimal odds, and multiply from there... to answer your question in this specific case...
-120 = 100/120 = 1.833
-135 = 100/135 = 1.741
1.833 X 1.741 = 3.19
So if the answer you're looking for is what a $100 bet would pay, the answer is roughly $220 ($320 - $100)
If the answer you're looking for is how to make these odds win $100, the answer is roughly $46 ($46 X 3.19 = $146.74, or a $100.74 profit)