If a car is travelling 80 miles per hour on a slippery road, then its stopping distance D in feet can be calculated by D=(6400/80)*(1/x), where x is teh coefficient of friction between the tires and road and 0

Question

If a car is travelling 80 miles per hour on a slippery road, then its stopping distance D in feet can be calculated by D=(6400/80)*(1/x), where x is teh coefficient of friction between the tires and road and 0<x< or equal to 1.
multiply and simplify the formula for D
and 
Compare the stopping distance on an icy road with x=0.2 and on dry pavement with x=0.8.

so
D=?
the stopping distance on an icy road is ?? feet.(round to the nearest whole number as needed)
the stopping distance on dry pavement is?? ft.

When a car is moving at 80 miles per hour, the stopping distance on dry pavement is__ of