FIrst calculate the speed, as follow:
speed = distance/time
distance = 2 1/4 mi = 2.25 mi
time = 3/4 h = 0.75 h
speed = 2.25 mi/0.75 h = 3 mi/h
next, for the distance traveled in 2 1/2 hours, use:
distance = speed x time
time = 21/2 h = 2.5 h
distance = (3 mi/h) x (2.5 h)
distance = 7.5 mi =7 1/2 mi
Hence, you can walk 7 1/2 miles in 2 1/2 hours