Question 621207
I would plot {{{p}}}, price/tile  on the vertical axis and
tiles sold/month on the horizontal
You are given 2 points on a straight line:
( 42, 20 )
( 52, 10 )
Use the point-slope formula
{{{ ( p - 10 ) / ( x - 52 ) = ( 20 - 10 ) / ( 42 - 52 ) }}}
{{{  ( p - 10 ) / ( x - 52 ) = 10 / (-10) }}}
{{{  ( p - 10 ) / ( x - 52 ) = -1 }}}
{{{ p - 10 = -x + 52 }}}
{{{ p = -x + 62 }}}
Here's a plot:
{{{ graph( 400, 400, -10, 100, -10, 100, -x + 62 ) }}}