The end product is totally wrong! Factorization is adding brackets but you removed the brackets! This is expansion...
The only thing I can deduce is that it was expanded not factorised.I think that whoever who did this should also improve on the handwriting. ._.
Handwriting should be neater as we are unable to read it.It should be written as [(a+3b)+(2c+d)][(a+3b)-(2c+d)], when simplified, its (a+3b+2c+d)(a+3b-2c-d)
He did not factorise it at the end answer, he expanded the question.
The final answer should be in the form of (a+b)(a-b)