Determine whether each trinomial is a perfect square. If so, factor. If not, explain. \text { 9. } x^{2}-6 x+9 \text { 10. } x^{2}-6 x-9 \text { 11. }

16 x^{2}+8 x+4 \text { 12. } 9 x^{2}-12 x+4 \text { 13. } 36 x^{2}-12 x+1