Please explain how we'd define our random variable and the possible values we can assign it to. Can there be negative values for the random variable X -like -3 to -1? Or do PDFs have to consist of positive values? (I'm not talking about probabilities, but the X-sub-i values)I realize c must be positive so that we can have real probability values. But the part that's bugging me is that our random variable isn't defined. Like what is our random variable of interest here?

4) You're a bidding for a government contract, and your only competition will bid with a PDF f(X) = c(9- x2) . How much should you bid so that you will get the contract with 90% probability