m+n=317
n+p=548
-------------
m+2n+p=?
(m+n)+(n+p)=m+2n+p
317+548=865
m+2n+p=865
m+p=865-2n
2n=(548-317)*2
2n=231*2
2n=462
n=462:2
n=231
m+n=317
m+231=317
m=317-231
m=86
n+p=548
231+p=548
p=548-231
p=317
m+2n+p=865
86+(2*231)+317=
=86+462+317
=548+317
=865
