Question 7336
that's a great problem. Start solving this system a little bit, it will help. Subtract 3 times the bottom equation, from top equation:

3x-my - 3(x+y) = 4-3*12
-my-3y = -32

remove minuses just to make it more readable

my+3y = 32

the value of m that makes this equation impossible (no solutions) is such am that makes the equation 0=32. 0=32 is an impossible equation.

-3 looks pretty good:

-3y+3y=32 is the same as 0 = 32.

so, the answer is m=-3.