.
Since the coefficients of the polynomial are real numbers (they even are integers (!) ),
it implies that together with the imaginary root 5i, its complex conjugate -5i is also the root.
Hence, the polynimial f(x) is divisible by (x-5i)*(x+5i) =
.
When you perform long division, you will get the quotient q(x) =
=
.
You can factor this quotient further using grouping/re-grouping
=
-
=
-
=
.
Therefore, the full decomposition of the given polynomial over complex number domain is
f(x) =
,
and its roots are 5i, -5i,
,
and -1. ANSWER
Solved.