Offrance 3 miles around the school track and then half the distance on a treadmill each day write an expression to show the number of miles paul runs in d days

The first thing we must do for this case is to find the number of miles traveled during a day:
 "3 miles around the school track and then half the distance on a treadmill"
 Thus:
 3 + (1/2) * (3) = 3 + 1.5 = 4.5 thousands
 Then, an expression for the number of miles is:
 f (D) = 4.5D
 
 an expression to show the number of miles Paul runs in D days is:
 f (D) = 4.5D