Let x=a+d where d is a very small positive value compared to a. We can write x^2-a^2 as (a+d-a)(a+d+a)=d(2a+d)=2ad, if we ignore d^2 as insignificant. sin(3x-3a)=sin3(a+d-a)=sin3d. When d is small sin3d is approximately 3d. So we have 2ad/3d=2a/3. As d approaches 0, x approaches a, therefore the limit is 2a/3. Note that this is the same limit if d is negative, because d cancels out. I think this word explanation is sufficient not to require a picture or expansion in MS Word.