Question 768564
The equation relating position (mile marker) to time driving hours) is the equation of the line that contains points (3,210) and (5,90).
{{{slope=(90-210)/(5-3)=-120/2=-60}}}
That is the speed (60mph), with a minus sign that means mile marker numbers are going down. 
The equation in point-slope form, based on point ((5,90) is
{{{y-90=-60(x-5)}}}
(We could also have written it based on point (3,210) as
{{{y-210=-60(x-3)}}}
From either form of the equation we could convert it to the slope-intercept form by solving for y. That would be the equation most useful in this situation.
{{{y-210=-60(x-3)}}}-->{{{y-210=-60x+180}}}-->{{{highlight(y=-60x+390)}}}
The y-intercept is {{{highlight(390)}}}, the mile marker where the driving at constant 60mph speed started.
The x-intercept can be calculated by making {{{y=0}}} in (any form of) the equation, and then solving for {{{x}}}.
For example, from the slope-intercept form,
{{{0=-60x+390}}}-->{{{60x=390}}}-->{{{x=390/60}}}-->{{{x=13/2}}}-->{{{x=highlight(6.5)}}}
In other words, {{{6.5hours=6hours}}}{{{30minutes}}} is the x-intercept, which means that the mile marker 0 will be reached after driving for that long.
{{{drawing(300,300,-2,8,-200,800,
grid(0),line(0,390,8,-90),
locate(6.5,60,"(6.5,0)"),locate(0,450,"(0,390)"),
locate(3,270,"(3,210)"),locate(5,150,"(5,90)"),
blue(circle(0,390,0.12)),blue(circle(3,210,0.12)),
blue(circle(5,90,0.12)),blue(circle(6.5,0,0.12))
)}}}