Answer
$-3$
Work Step by Step
Given that we know $y=0$ is part of the solution set,
we back-substitute it into one of the initial equations:
$6x+y=-18$
$6x+0=-18$
$6x=-18\quad/\div 6$
$x=-3$
Confirm by substituting into the second equation,
$13(-3)+(0)=-39.$
