Solution :
To factor completely

To factor it completely , first take the product of first and third term, and then break the second term in two parts in such a way that its sum equals the second term and the product equals the product of first and third term.
Product of first and third term is

Second term is 3a, Re-write 3a as sum of 7a and -4a.
Lets check it Product of 7a and -4a is
wich is eaqual to product of first and third term and sum of 7a and -4a is 3a which is the second term.
Now factorise, we get

Hence,
is the factor of
.