Just look at the left hand side: . We need to figure out which number to add/subtract such that we can then factorise it as or
so, expanded would give and the negative version would have -2ax instead.
So, we have -6x which is the (-2ax) which means that a needs to be 3.
So, the term is ---> 9
So, we have to add 9...to both sides remember, to keep the equation unaltered...
So, can be written instantly as --> the negative version, since the x-term was negative.