Question 492864
I will plot boxes on the x-axis and cost/box on the y-axis
Us the point-slope formula, which, in general is:
{{{ ( y - y[1] ) / ( x - x[1] ) = ( y[2] - y[1] ) / ( x[2] - x[1] ) }}}
In this case, {{{ x = B }}} and {{{ y = P }}}
The given points are:
(3,200) and (4,120)
{{{ ( P -  120 ) / ( B - 4 ) = ( 200 - 120 ) / ( 3 - 4 ) }}}
{{{ ( P - 120 ) / ( B - 4 ) = 80 / -1 }}}
Multiply both sides by {{{ B - 4 }}}
{{{ P - 120 = -80*( B - 4 ) }}}
{{{ P = -80B + 320 + 120 }}}
{{{ P = -80P + 440 }}}
check:
Does it include the given points?
(3,200)
{{{ P = -80P + 440 }}}
{{{ 200 = -80*3 + 440 }}}
{{{ 200 = -240 + 440 }}}
{{{ 200 = 200 }}}
(4,120)
{{{ 120 = -80*4 + 440 }}}
{{{ 120 = -320 + 440 }}}
{{{ 120 = 120 }}}
OK