The break even point is where the two lines intersect. You'll need to graph I = 80n as well as C = 150 + 2n.
You could plot some points.
For cost, C = 150 + 20n
When n = 0, C = 150
When n = 1, C = 170
When n = 2, C = 190
When n = 3, C = 210, and so on.
For income, I = 80n
When n = 0, I = 0...