A lot of math is about matching patterns. Here, the pattern you can match is given in the problem statement:
d = 6t . . . . . equation for distance traveled at 6 miles per hour
You are asked to write an equation for distance traveled at 8 miles per hour. You can see the number 6 in the above equation matches the "miles per hour" of the traveler. This should give you a clue that when the "miles per hour" changes from 6 to 8, the number in the equation will do likewise.
The equation you want is ...
d = 8t . . . . . equation for distance traveled at 8 miles per hour
