Because the random variable X is uniformly distributed between 10 and 15 then the probability of a bid's acceptance is 0 if X is less than or equal to 10. On the other hand, if the bid was greater than or equal to 15 then the probability of acceptance would be 1.
Values of X between 10 and 15 will have probabilities between 0 and 1. So, for example if X=12 (X=14) than the probability of acceptance would be p=0.40 (.80).
I suspect you can take it from here.