Question 1016709
Plot price, {{{ P }}} on the vertical axis
Plot number of calculators/wk sold, {{{ n }}}
where {{{ n }}} is in thousands
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You are given the points:
( 10, 95 )
( 12, 90 )
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Use the general point-slope formula
{{{ ( P - 90 ) / ( n - 12 ) = ( 95 - 90 ) / ( 10 - 12 ) }}}
{{{ ( P - 90 ) / ( n - 12 ) = 5 / (-2) }}}
Multiply both sides by {{{ -2*( n - 12 ) }}}
{{{ -2P + 180 = 5n - 60 }}}
{{{ 2P = -5n + 240 }}}
{{{ P = -(5/2)*n + 120 }}} answer 
( change {{{ P }}} to {{{ x }}} if you like )
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check answer:
( 10,95 )
{{{ P = -(5/2)*n + 120 }}}
{{{ 95 = -(5/2)*10 + 120 }}}
{{{ 95 = -25 + 120 }}}
{{{ 95 = 95 }}}
and
( 12,90 )
{{{ 90 = -(5/2)*12 + 120 }}}
{{{ 90 = -30 + 120 }}}
{{{ 90 = 90 }}}
OK