Multiplicity is how many times the root repeats
roots r1 and r2 of a polynomial factor to
(x-r1)(x-r2)
so 3 multiplicty 2 means (x-3)^2 is in the factorization of that polynomial
also, for a polynomial with real coefients, if a+bi is a roots, a-bi is also a root
5i is a root, therefor -5i is a root as well
roots are
(x-5i)(x+5i)(x-3)^2
if we expand
x^4-6x^3+34x^2-150x+225
the polynomial is
f(x)=x^4-6x^3+34x^2-150x+225