total road length is 36 miles.
If one crew’s rate is x miles/day, the other is 1.4x miles/day.
The collective distance per day is x+1.4x,
so in five days the distance is 5(x+1.4x).
Set that equal to the total distance:
5(x+1.4x)=36
simplify:
x+1.4x=7.2
x(1+1.4)=7.2
x(2.4)=7.2
x=3