Question 192028
factoring  is  recognition  of  patterns

a^2-B^2  =  (a+b)  (a-b)  

9C^2 -121=  (3c)^2 -(11)^2= (3c+11)(3c-11)

checking  by  foil

(3c+11)(3c-11)

9c^2 -33c+33c-121=  9c^2-121