Solve the Problem $6 \frac { 4 } { 7 } \mathrm { mph }$

Solve the problem.
-Gary can hike on level ground 3 miles an hour faster than he can on uphill terrain. Yesterday, he hiked 28 miles, spending 2 hours on level ground and 5 hours on uphill terrain. Find his average speed on level ground.

A) $6 \frac { 4 } { 7 } \mathrm { mph }$
B) 4 mph
C) $3 \frac { 1 } { 7 } \mathrm { mph }$
D) $6 \frac { 1 } { 7 } \mathrm { mph }$