If his normal speed is v mph then the distance in miles is x=vt where t is the time in hours to complete the journey. Since x=140, vt=140, so t=140/v hours.
If his speed changes to v=v+5, then (v+5)(140/v-0.5)=140, because the time is reduced by 0.5 hours.
140-0.5v+700/v-2.5=140,
-0.5v+700/v-2.5=0,
-0.5v2+700-2.5v=0.
Multiply through by -2:
v2+5v-1400=0=(v-35)(v+40)
v=35mph.
CHECK
Journey time = 140/35=4hr.
5mph more than usual=40mph. Time to travel 140 miles=140/40=3.5hr, which is half an hour shorter than normal journey time.