The distributive property of multiplication over addition can be used when you multiply a number by a sum. The distributive property lets you multiply a sum by multiplying each addend separately and then add the products:
The distributive property of multiplication over subtraction can be used when you multiply a number by a difference. The property states that the product of a number and the difference of two other numbers is equal to the difference of the products:
Therefore, you can see what sign is in the brackets, if sign is "+", then you have to add, if sign is "-", then you have to subtract.