If we take 42/4, or any other of the sets, you will get 10.5, which is k.
10.5 is the amount of miles travelled in 1 hour.
Since we just multiply time and miles to get y, our equation will just be in y=kx format. 10.5 * time is the same as writing 10.5x since x is the hours. y would be miles.
Constant of proportionality: k = 10.5
The equation: y = 10.5x
:)