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
When n = 1, I = 80
When n = 2, I = 160
When n = 3, I = 240, and so.
We can see that I gets bigger than C after n = 2. Let's try n = 2.5.
When n = 2.5, C = 150 + 20(2.5) = 150 + 50 = 200
When n = 2.5, I = 80(2.5) = 200
So break even point is (2.5,200), or in other words, where the two lines intersect.