What does “m” stand for in the slope formula?

Iam so stuck. what is the worked out answer for this question?

Malia is observing the velocity of a cyclist at different times. after two hours, the velocity of the cyclist is 15 km/h. after five hours, the velocity of the cyclist is 12 km/h. part a: write an equation in two variables in the standard form that can be used to describe the velocity of the cyclist at different times. show your work and define the variables used. (5 points) part b: how can you graph the equations obtained in part a for the first 12 hours? (5 points) if you can try and make this as little confusing as you can

The water has been on the stove y=9.5x+62.1 after how many minutes will the tempature be 138.1 f