[tex]9)\\\left\{\begin{array}{ccc}y=-3\\2x-3y=15\end{array}\right\qquad\text{put the value of y to the second equation}\\\\2x-3(-3)=15\\2x+9=15\qquad\text{subtract 9 from both sides}\\2x=6\qquad\text{divide both sides by 2}\\\boxed{x=3}\\\\Answer:\ x=3,\ y=-3\to(3,\ -3)[/tex]
[tex]10)\\\left\{\begin{array}{ccc}x=5\\3x-y=10\end{array}\right\qquad\text{Put the value of x to the second equation}\\\\3(5)-y=10\\15-y=10\qquad\text{subtract 15 from both sides}\\-y=-5\qquad\text{change the signs}\\\boxed{y=5}\\\\Answer:\ x=5,\ y=5\to(5,\ 5)[/tex]
[tex]11)\\\left\{\begin{array}{ccc}y=4x-1&(*)\\3x-y=-2\end{array}\right\\\\\text{Substitute}\ (*)\ \text{to the second equation}\\\\3x-(4x-1)=-2\\3x-4x-(-1)=-2\\-x+1=-2\qquad\text{subtract 1 from both sides}\\-x=-3\qquad\text{change the signs}\\\boxed{x=3}\\\\\text{Put the value of x to}\ (*):\\\\y=4(3)-1\\y=12-1\\\boxed{y=11}\\\\Answer:\ x=3,\ y=11\to(3,\ 11)[/tex]
