Question 512851
What they are really asking is for the graph of the function
{{{ f = g(t) }}}. The monthly charge would be the slope of the line
if I plot f versus months (t) 
Use the point-slope formula
{{{ ( f - 75 ) / ( t - 3 ) =  ( 115 - 75 ) / ( 5 - 3 ) }}}
{{{ ( f - 75 ) / ( t - 3 ) = 40 / 2 }}}
{{{ ( f - 75 ) / ( t - 3 ) = 20 }}}
{{{ f - 75 = 20*( t - 3 ) }}}
{{{ f = 20t - 60 + 75 }}}
{{{ f = 20t + 15 }}}
So, the monthly charge is $20 and the set fee is $15
for 8 months:
{{{ f = 20*8 + 15 }}}
{{{ f = 160 + 15 }}}
{{{ f = 175 }}}
Aaron pays $175