Fill in the blank with a constant, so that the resulting expression can be factored as the product of two linear expressions:
2mn - 21m + 6n + mn - 9m + ___
3mn - 30m + 6n
3m ( n - 10) + 6 (n -10)
(n -10) ( 3m + 6)
3mn -30m + 6n + - 60
+1348 
