Answer:
Constant of proportionality: 10.5 or k= 10.5
y= 10.5x
Step-by-step explanation: You will take your y/x value, which in this case is miles per hour. So you will do each of your miles per hour.
21/2= 10.5
31.5/3= 10.5
42/4= 10.5
Therefore your constant of proportionality is 10.5.