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The distributive law says that:

a(b + c) = ab + ac

Suppose you have a problem like:

-4 - 5

What does it have to do with the distributive law? Well,

-4 - 5

= (-4) + (-5)

= (-1)(4) + (-1)(5)

This is like ab + ac with a = -1. Now we can factor out the -1 to get:

(-1)(4 + 5)

which is same thing as

-(4 + 5) = -9

because the 1 is understood when we write it this way.