Answer:
Step-by-step explanation:
[tex]\left\{\begin{array}{ccc}y&=&-2x\\y&=&-9x+21\\\end{array} \right.\\\\\\\left\{\begin{array}{ccc}y&=&-2x\\-2x&=&-9x+21\\\end{array} \right.\\\\\\\left\{\begin{array}{ccc}y&=&-2x\\9x-2x&=&21\\\end{array} \right.\\\\\\\left\{\begin{array}{ccc}7x&=&21\\y&=&-2x\\\end{array} \right.\\\\\\\left\{\begin{array}{ccc}x&=&3\\y&=&-6\\\end{array} \right.\\\\\\[/tex]
