You're right so far, and I think you've done the hardest bits! All you need to do for the final part is to put the expression you have for part 2 equal to , and solve the equation for . So you get:
(not , because must be positive)
So the jogger runs at mpg uphill and mph downhill.
Just in case you think that answer is wrong, and would give an average speed of mph, just try it out.
The time taken to run 1 mile at 3 mph = hours, and at 9 mph is hours. So the total time taken to run the 2 miles is hours. So the average speed is mph