[tex]\left\{\begin{array}{ccc}2x+y-c=2\\x+2y-c=5\\x-y+2c=-7\end{array}\right\ |\text{add both sides of equations}\\4x+2y=0\ \ |-4x\\2y=-4x\ \ |:2\\y=-2x\\\text{substitute y=-2x to thefirst and third equation}\\ \left\{\begin{array}{ccc}2x+(-2x)-c=2\\x-(-2x)+2c=-7\end{array}\right\\\left\{\begin{array}{ccc}-c=2&\to c=-2\\3x+2c=-7\end{array}\right\\\text{substitute the value of c to the second equation}\\3x+2(-2)=-7\\3x-4=-7\ \ \ |+4\\3x=-3\ \ \ |:3\\x=-1\to y=-2(-1)=2\\\\\text{Answer:}\ x=-1;\ y=2;\ c=-2[/tex]
