A(-1; 7; 2), B(-1; 3; 4), C(1; 2; 9) m=AB+2BC

AB={0;-4;2}, BC={0;-1;5}, 2BC={0;-2;10}m=AB+2BC={0;-4;2}+{0;-2;10}={0;-6;12}|m|=sqrt(0^2+36+144}=sqrt(180)=6sqrt(5)