Answer:
The polynomial expression x^4 + 4x - 21 cannot be factored into linear factors with rational coefficients. This can be shown using the rational root theorem, which states that any rational roots (zeros) of the polynomial must be of the form p/q, where p is a factor of the constant term (in this case, 21) and q is a factor of the leading coefficient (1). The only possible rational roots of this polynomial are ±1, ±3, ±7, and ±21. However, plugging in each of these values does not give a zero of the polynomial, meaning it has no rational roots. Therefore, it cannot be factored into linear factors with rational coefficients. It may still be factorable using complex numbers, but it is not factorable using only real numbers.