Descartes rule of signs gives partial information. The rule says the number of real positive roots is less than or equal to the number of sign changes. For a polynomial with real coefficients the complex roots (which are not real) appear in conjugate pairs i.e., if a+ib
a
+
i
b
is a root then a−ib
a
−
i
b
is also a root. Since degree of polynomial is five therefore it has at least 1 real root. Thus number of complex root(s) (which are not real) is either 0, 2 or 4.