Question 180671
There are 2 equations and 2 unknowns,
so it is solvable. If there were more
unknowns than equations it would be 
unsolvable
(1) {{{2m + n = 1}}}
(2) {{{m - n = 8}}}
Just add the equations
{{{2m + m + n - n = 1 + 8}}}
{{{3m = 9}}}
Divide both sides by {{{3}}}
{{{(3m)/3 = 9/3}}}
{{{m = 3}}}
As long as I do the same thing to both 
sides, the equation is still true.
Put {{{m = 3}}} back into (2)
{{{3 - n = 8}}}
Subtract {{{3}}} from both sides
{{{-n = 8 - 3}}}
{{{-n = 5}}}
Multiply both sides by {{{-1}}}
{{{n = -5}}}
The solution is {{{m = 3}}} and {{{n = -5}}}