The result is9a2β16The reason is the following:The problem is an example of a notable product: "the sum multiplied by the diference is equal to the difference of squares", that is to say:(a+b)β (aβb)=a2βb2.By applying this to our question, we obtain that:(3aβ4)β (3a+4)=(3a)2β(4)2=9a2β16.