Question 19323
z^2+25=...since we are dealing with complex numbers we can write +25=-(25)*(-1)
=-(5*5)(i*i)=-(5i)^2...so we have.... z^2+25 =z^2-(5i)^2
using the formula (x+y)(x-y)=x^2-y^2..we get 
z^2-(5i)^2=(z+5i)(z-5i)