Mathematical Error in understanding a Math problem

In several days ago, I read a page on Yahoo Answers. How surprised when I got a question and its answer. Questions and answers can be viewed here.

Most of the people or the students have little ability to understand the matter in Mathematics appropriately. Usually they make a mistake in a matter of understanding mathematics. This becomes a heavy task for the next prospective Mathematics teachers. Mathematics teachers have great responsibility to convey the concept of Mathematics to be a clear and precise.

One of the mistakes in the answers to the question asker is in the following:

sebenernya sech jwbnnya ga sesuai dgn gue mo.jwbn gue gini :
( a2 - a2 ) = (a2 - a2 )
a( a - a ) = (a + a ) ( a – a )
a = 2 ( a – a )
( a – a )
"a-a"atas + bwh di coret,maka
a = 2a
= 2
1 = 2
tvi ga pa2 lah yg pntng jwbnnya sma,Anda emang orang pintar !!!

In the answer above, the person who gave answers to forget that a-a = 0. He forgot that the division with the number 0 is not defined.

So, as a good Mathematics teacher, he/she should be able to give a clear explanation about the Mathematics concept to the students , so that the error does not occur as above.