Level the floor

We've just moved.  The audio playpen is the basement, which has an issue I haven't dealt with before: uneven floor.  Left-right is level, but front to back there's a steep slope.  At a distance of 18 inches, the slope is about one inch.

I figure others have dealt with this enough that there's "a way" (or an eternal argument...).  Educate me?

Available equipment would be fine; surprise uses of stuff would be terrific.

Thanks.