m - n
m = 300x + 100, so we substitute that into the equation for m.
300x + 100 - n
n = 250x + 200, so we substitute that into the equation for n.
300x + 100 - 250x - 200
Subtract the 250x to get:
50x + 100 - 200
Subtract the 200 to get:
50x - 100, which is option D.
Hope this helps!