mn - 4m - 5n + 20
In this, mn and -4m have a common factor; m. So we can take that out to make it:
m(n-4) -5n + 20
Now, -5n + 20 also has a common factor which is 5, because 20/5 gives you 4, and 5/5 equals 1. So we can talk out that -5 (it doesn't need to be negative, but negatives on variable, no gusta). So you'd get
m(n-4)-5(n-4) OR m(n-4)+5(-n+4)
