Use y2-y1 divided by x2-x1 to find slope and then plug that into y-y1=m(x-x1) so you should get 50,000-20,000 divided by 300-100 and you should get 150 for the slope. Plug that in and you get that
y-20,000=150(x-100)
y-20,000=150x-15,000
y=150x+5,000 is the equation for (100, 20,000)...