Are we to assume that the 100 students who graduated were among those who ordered yearbooks? But that, because they graduated before the yearbooks arrived they did not pay for them? If not, I see no way to do this problem.

Assuming those things, let the number of students who ordered year books be "x". If they all paid for year books, they would have to pay dollars each. Because 100 of then are not paying for the yearbooks, there are x- 100 who are and each will have to pay . We are told that that second cost is "an additional $6". That is, we have

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That is the equation you need to solve. I recommend multiplying both sides by x(x-100).