5x - 2y = 12
3x - y = 4
solve for y;
3x - y = 4
-y = 4 - 3x
y = -4 + 3x
y = 3x - 4
substitute "y = 3x - 4" into the first equation;
5x - 2y = 12
5x - 2(3x - 4) = 12
8 - x = 12
solve for x in "8 - x = 12";
8 - x = 12
-x = 12 - 8
-x = 4
x = -4
solve for y given the x value in the second equation;
y = 3x - 4
y = 3(-4) - 4
y = -16
x = -4, y = -16
hope this helps, God bless!