m(a)/(a+1)=n-3/(a+1), where n is an integer, EXCEPT when a+1=0, that is, a=-1.
We can say that a cannot be -1 because that would give us division by zero.
Your statement is not correct. It should be “variable a cannot be -1 because that would mean a+1 is zero and dividing by zero gives an undefined result”.
(For example, if m(a)=a-2 then m(a)/(a+1)=(a-2)/(a+1)=1-3/(a+1). Now put a=-2, and we get the result 4. Put a=0 and the result is -2. Put a=-1.1 and the result is 31; for a=-0.9 it’s -29.)