$2x=8$
Divide both sides of the equation by $2$$x=4$
Substitute the given value of $x$ into the equation $x+y=-1$$4+y=-1$
Solve the equation for $y$$y=-5$
The possible solution of the system is the ordered pair $\left( x, y\right)$$\left( x, y\right)=\left( 4, -5\right)$
Check if the given ordered pair is the solution of the system of equations$\left\{\begin{array} { l } 4+\left( -5 \right)=-1 \\ 4-\left( -5 \right)=9\end{array} \right.$
Simplify the equalities$\left\{\begin{array} { l } -1=-1 \\ 9=9\end{array} \right.$
Since all of the equalities are true, the ordered pair is the solution of the system$\left( x, y\right)=\left( 4, -5\right)$