The polynomial has a degree of , whose coefficients are real numbers.

Its zeros are and .

By conjugate pairs theorem:

If is a zero of the polynomial then is also be the zero of the function.

Thus, third zero of the polynomial is .

If the fourth zero is a complex number then its conjugate is also zer of the function which tends to the degree of the

polynomial as .

Therefore, the fourth zero must be a real number only.

Missing zero of the polynomial is .

Fourth zero must be a real number only.