15. b^2-c^2-b^2+2bc=-c(c-2b)
16. a^2-c^2-3ac+c^2=a(a-3c)
17. a^2+5ab-a^2+b^2=b(5a+b)
18. 3ab-b^2-a^2+b^2=a(3b-a)
19. y^2-2y+10y-20-8y+12y^2=13y^2-20
20. a^2+9a-4a-36-5a+10a^2= 11a^2-36
21. 6b^2+4b-9b-6-6b^2-9b= -14b-6= -2(7b+3)
22. 6a^2-9a-2a+3-6a^2-10a= 3-21a=3(1-7a)