Explanation:
To answer your question, I will give two examples.
First and foremost, distributive property of a system of linear expression in algebra allows us to simply an expression by following the rule of expansion.
now, to know whether to add, lets take a look at this example;
a(b+c) = ab + bc
simplifying the expression above gives ab + bc
this shows that when you are using the distributive property to multiply an addition, you will add the result.
on the other hand, lets look at ; a(b-c) = ab - bc
the result shows that when you are using the distributive property to multiply a difference of variables or number, you will subtract the result.