How to factor by grouping

Ok first thing your gonna want to do there is factor out the gcf, the greatest common factor, which in this case, is 3, you then get 3(7k^3-28k^2+5k-20). you then want to ignore your 3 and group some of your terms together so if you group them together you can take (7k^3 - 28k^2) and (5k-20) you then gcf those so you get 7k^2( k - 4) and 5(k-4)  you notice that both have k-4? they should always both have the same thing inside the parentheses like that. now you take (7k^2+5)(k-4) this will sometimes be your ending point, but we cant forget aobut the gcf in the beginning. your final answer is 3(7k^2+5)(k-4)

These are the multiples of 15 but what you are probably asking or what are the factors of 15