Answer:
Third zero is -1.
Step-by-step explanation:
consider the given polynomial is
It is given that -1 and -3 are two zeroes, therefore (x+1) and (x+3) are two factors of given polynomial.
The given polynomial can be rewritten as
Now, splitting the middle term, we get
Here, power of (x+1) is 2. It means the multiplicity of zero -1 is 2.
So, three zeroes are -1, -1 and -3.
Therefore the third zero is -1.
