They have already traveled 20 miles, so the starting point at x=0 hours is y(0)=20 miles.
The constant rate of 50 mi/h is the slope that relates the variation of the distance y with the variation of time x. As we have a constant rate, we have a linear relationship and this rate is called slope.
Then, we have the y-intercept of the line y(0)=20 and the slope m= 50 mi/h.
We can write the function in slope-intercept form as:
Answer: y = 50x + 20