It is just the multiplication of vectors. We'll multiply corresponding pairs together. The general formula for the pairs of ([tex] x_{1} [/tex]; [tex] y_{1} [/tex]) and ([tex] x_{2} [/tex]; [tex] y_{2} [/tex]) is [tex] x_{1} [/tex] * [tex] x_{2} [/tex] + [tex] y_{1} [/tex] * [tex] y_{2} [/tex] = (-6)*(-3)+4*6=18+24=42