We are looking for the point at which the compensation from Plan A is less than the compensation from Plan B.
Plan A = 850 +0.1x
Plan B= 1050+(x-7000)(0.14)
Plan A < Plan B
850 +0.1x <1050+(x-7000)(0.14)
then we sill simplify
850+0.1x < 1050+0.14x-980
850+0.1x<70+0.14x
then we isolate the x
0.1x-0.14x<70-850
-0.04x<-780
x>-780/-0.04
x>19500
In this case the monthly sales x need to be greater than 19500 (x>19500), in order that Plan B were better than plan A.