Question 352558
Let {{{C}}} = cost, and plot on vertical axis
Let {{{x}}} = number of chairs, and plot on
horizontal axis
You are given 2 points on the graph, 
(700, 20000) and (1500, 30000)
Use the point-slope formula
{{{(C - C(1))/(x - x(1)) = (C(2) - C(1)) / (x(2) - x(1))}}}
{{{(C - 20000)/ (x - 700) = (30000 - 20000) / (1500 - 700)}}}
{{{(C - 20000)/ (x - 700) = 10000 / 800}}}
{{{(C - 20000)/(x - 700) = 12.5}}}
{{{C - 20000 = 12.5x - 8750}}}
{{{C = 12.5x + 11250}}}
T make 2000 chairs:
{{{C = 12.5*2000 + 11250}}}
{{{C = 25000 + 11250}}}
{{{C = 36250}}}
It cost $36,250 to make 2000 chairs