Question 107413
(b-c) is the common factor. Pull in out to the front

{{{(b-c)*(a^2-16b^2)}}}
{{{(a^2-16b^2)}}} is a difference of two squares so it factorises to 
{{{(a+4b)(a-4b)}}}
we now have
{{{(b-c)(a+4b)(a-4b)}}}