Answer :
[tex]3n = -18\\
3m - 2n + 4p = 12\\
2m - 4n = 16\\\\
n=-6\\
3m-2n+4p=12\\
m-2n=8\\\\
3m-2\cdot(-6)+4p=12\\
m-2\cdot(-6)=8\\\\
3m+12+4p=12\\
m+12=8\\\\
3m=-4p\\
m=-4\\\\
3\cdot(-4)=-4p\\
-4p=-12\\
p=3\\\\
\boxed{(m,n,p)=(-4,-6,3)}[/tex]