(-) + (-) = (-)
(-) - (-) = solve the equation like (-) + (+)
(+) + (+) = (+)
(+) - (+) = (+) or (-)
(+) + (-) = solve the equation like (+) - (+)
(+) - (-) = solve the equation like (+) + (+)
(-) + (+) = (+) or (-)
i think i covered them but tell me if i missed sum.