Aschool club is raising money for a trip, and needs to reach $10,000. their fundraising progress is modeled by the function

f(x) = 435 + 1200x, where x is measured in weeks.

what is the meaning of the coefficient 1200?

Well lets determine what is the significance and also lets solve the problem: 
The constant in here is = 435 which is the initial capital fund .
The amount collected in a week would be = 1200.That answers your question Therefore in x weeks, with weekly increasing 1200$ and the fixed 435$ then 
the goal will be reached in 8 weeks or so. I hope this info is useful for you.