Answer: 9 (x^2 -5y^4) (x^2+5^4) 
Explanation:
Hi, to solve this we have to analyze the equation. 
9x^4 -225 y ^8 
First, we have to remove the common factor (9) 
So, mathematically speaking: 
9 (x^4 -25y^8) 
The next step is to factor the difference of two perfect squares 
9 (x^2 -5y^4) (x^2+5^4) 
Feel free to ask for more if it´s necessary or if you did not understand something.