$$3 \cdot \vecAM = \vecAB$$ $\vecAM = \left( x-(-7), y-2 \right) =(x+7,y-2)$ $\vecAB = \left( 8-(-7), 8-2 \right) =(15,6)$ $$3 \cdot \vecAM = \vecAB$$ $$3 \cdot (x+7,y-2) = (15,6)$$ $$(3x+21,3y-6) = (15,6)$$ Igualamos componente a componente $3x+21= 15 \longrightarrow 3x=-6 \longrightarrow (…)