.
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.