a plus times a plus is a plus.
a plus times a minus is a minus.
a minus times a minus is a plus.

Works just as well as:

a plus times a plus is a plus.
a plus times a minus is a plus.
a minus times a minus is a minus.

(-a)(-b) = (-1)(-a)(-1)(-b) = (-1)(-1)(-a)(-b)
= (-a)(-b) = (-1)(-ab) = -ab = -ab

(-3 lots of) 5
-5-5-5
-3 (lots of 5)
555