So, we have to factor completely the expression 9k²-121m².
We have a special product that is:
(a+b)(a-b)= a²-b²
So, we need to find a and b so that a² is 9k² and b²=121m²
sqrt(9k²) is 3k and sqrt(121m²) is 11m. Therefore:
(3k+11m)*(3k-11m)= 9k²-121m²
