Question 884300
{{{ y = m*x + b }}}
The slope will be their average speed, {{{ s }}}
Let {{{ d }}} = the distance from Salt Lake City
{{{ d[1] }}} = the distance they have already traveled
The equation looks like:
{{{ d = s*t + d[1] }}}   
Set {{{ t = 0 }}} to find {{{ d[1] }}} 
{{{ d[1] = 507  }}}
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Between 8 hrs and 4 hrs of driving, they have been
driving 4 hrs and have traveled {{{ 507 - 267 = 240 }}} mi
{{{ s = 240/4 }}}
{{{ s = 60 }}}
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I come up with:
{{{ d = -60t + 507 }}}
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I made the slope negative because {{{ d }}} has to get smaller
as {{{ t }}} increases
Here's a plot:
{{{ graph( 400, 400, -1, 10, -50, 600, -60x + 507 ) }}}
It looks like they get to Salt Lake City about 8.5 hrs since they 
were 507 miles away ( at {{{ t=0 }}} )
Hope this helps