Answer:
The Missouri Compromise prevented them from using slaves in the area until the passing of the Kansas-Nebraska Act.
Step-by-step explanation:
the Kansas-Nebraska Act in 1854 basically reversed the Missouri Compromise which would have prevented using slavery in Areas in the territory west of etc. The Southerners were angry basically because it would cost them a vast amount more money and the Missouri compromise had favored the Northerns as they used factories and thus did'nt need to spend that much of a difference to make good use of the land which southerners felt was unfair.