Let s = salary, and x = sales.
Plan 1 : s = 300 + .10x
Plan 2 : s = 1200
You are asked for the amount of money when the first plan will make more money.
Plan 1 > Plan 2
300 + .10x > 1200
.10x > 900 (subtract 300 from both sides)
x > 9000 (divide both sides by .10)
Therefore your sales must be greater than $9000 each month in order for Plan 1 to make more money.