How do you work out how to times a positive number by a negative number?

A positive number times a positive number will always get you a positive number

A negative number times a negative number will always get you a positive number

(same signs will get you a positive number)

but

A positive number times a negative number will always get you a negative number

or

A negative number times a positive number will always get you a negative number

(different signs will get you a negative number)