case A)
row 7 mi ------> speed=distance /time
time=distance/speed
time=7/2.1=3.33 hours
walk 13 mi
time=13/4=3.25 hours
total time=3.33+3.25=6.58 hours
case B
Find out the hypotenuse of the right triangle
c^2=7^2+13^2
c^2=218
c=14.76 mi
row 14.76 mi
time=14.76/2.1=7.03 hours
therefore
In order to make the fastest possible time ----> row 7 miles and walk 13 miles
total time=6.58 hours
rounded ----> 7 hours