Divide the first equation by 2:
[tex]\begin{cases}2x+y=0\\3x-y=-10\end{cases}[/tex]
Sum the two equations:
[tex]5x = -10[/tex]
Divide both sides by 5:
[tex]x=-2[/tex]
Substitute this value for x in one of the equation (for example the first) and solve for y:
[tex]-8+2y = 0 \iff 2y=8 \iff y=4[/tex]
