Question 716290
Set your D=RT box chart. Hard to illustrate here but attempt:
           
............RATE......TIME
1st group: 1.5........x
2nd group: 2..........x
Total distance is 21miles. Since you have been provided a total distance in the problem...you sum the R*T of both groups and set them equal to your total distance (21miles)

If D=RT, then multiply each groups rate by time respectively.

1.5x + 2x = 21
SIMPLIFY
3.5x=21
DIVIDE BOTH SIDES BY 3.5 TO GET 'X' ALONE
x=6

Always remember when writing your solutions to include the units. To check your work plug your solution back into the D=RT equation and it should = your total miles provided.