If x is the number of units, 600x dollars is the cost of manufacturing all the units, and 900x dollars is the revenue of the sales.
Break even is when the cost=revenue.
200000+600x=900x. 300x=200000, x=200000/300=2000/3=667 approx. There is a small profit of $100 but 666 units actually incurs a loss of $200. The break-even seems to be 667 units.