It is not safe for you to get a trampoline on land that is slanted. This could turn out and be fatal in case an accident should happen. Before getting a trampoline, there are things to consider such as proper drainage, sinking and the chances of cave in occurring. In your case, it is very likely for a cave in to happen.
You could buy some dirt/sand and put it on the ground where you want to put the trampoline so it would be more level. You would have to make sure you "pack" the dirt/sand down before putting the trampoline there. But it's not a good idea to jump on one when it is slanted.