So,
He rides 25 miles in 2 hours, 37.5 miles in 3 hours, and 50 miles in 4 hours.
(I'm thinking) the constant of proportionality is the same as solving for the unit rate.
Divide 25 by 2.
12.5 mph

That (I think) is the constant of proportionality
Now we will write an equation to represent all the data.

Hope I helped! If not, you can delete my answer.