If you know how much each class is selling tickets for and how much they've already raised then you take those two values and set them equal to each other.
<em>2.5t + 350 = 3t + 225</em>
To actually find the number of tickets each class would have to sell you would just solve for t.
Not really sure what you mean by compare. The only thing I can think of is the y intercept is generally a good point to start from to count the rise/run
