Question 263367
fist find for one variable
{{{9m+n=24}}}__{{{n=24-9m}}}  
{{{m-3n=12}}}
pick one equation and plug it in the other
so: {{{n=24-9m}}} 
{{{m-3(24-9m)=12}}}
{{{m-72+27m=12}}}
{{{-72+28m=12}}}
{{{-72+28m+72=12+72}}}
{{{28m=84}}}
{{{m=84/28}}}
{{{m=3}}}
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{{{m-3n=12}}}
{{{3-3n=12}}}
{{{3-3n-3=12-3}}}
{{{(-3n)/-3=9/-3}}}
{{{(cross(-3)n)/cross(-3)=9/-3}}}
{{{n=-3}}}