3x + y = 9 (I) y = –4x + 10 (II) ------------------------ [tex] \left \{ {{3x + y = 9 } \atop {y = -4x + 10 }} \right. [/tex]
Pass the incognito "4x" to the first term, changing the signal when changing sides. ------------------------- simplify by (-1) [tex]\left \{ {{3x + y = 9.(-1)} \atop {4x + y = 10 }} \right. [/tex]