confusing but this is the scenario
actor a is charging 250+50x where x=number of days
actor b is charging 200+60x (guessing that you meant to put + because a - sign wouldn't make sense)
so when are they equal?
set them equal to each other
250+50x=200+60x
subtract 50x from both sides
250=200+10x
subtract 200 from both sides
50=10x
divide both sides by 10
5=x
at 5 days, they will charge the same ammount