[tex]\left\{\begin{array}{ccc}2x-y=-20&(1)\\y=-2x&(2)\end{array}\right\\\\\text{Substitute (2) to (1):}\\\\2x-(-2x)=-20\\2x+2x=-20\\4x=-20\qquad\text{divide both sides by 4}\\x=-5\\\\\text{Put the value of }\ x\ \text{to (2):}\\\\y=-2(-5)\\y=10[/tex]
