Compute time to sell (with 50% probability) at Price P from linear regression

#1
I think this is a very basic question (from a total stat newbie). I'm trying to understand a problem I was given (I'm not a statistics student). It's a multi-part problem having to do with car sales. In the first part, I ran a linear regression on car mileage/sales price data (price being the dependent variable). This gives me a function where I can plug in a mileage number and it will spit out a price.

The next part of the problem states "using your results from the linear regression, compute an expected time to sell given 3 prices - [P, P+10%, P-10%] and assuming a 50% sell probability" and I'm not sure what this means or how to go about it. I suspect this wording is less vague to people who are well-versed in statistics. Could someone shed some light on it for me? Thanks!