I assume your "reasons" mean "roots" or "zeros"
if 3 is a root, plug x=3 in the equation, you can find n:
3²+(n-1)*3+6=0
3(n-1)=-15
n-1=-5
n=-4
Plug n=-4 in the original equation: x²-5x+6=0
factor: (x-3)(x-2)=0
x=3, which we already know
or x=2, which is the value of m+4
m+4=2
m=-2
