Subtract from either side. See if that leads anywhere helpful...?
...where is the group's identity element. The result follows immediately.
thx for the reply and I can understand the second part and for the first part if I takeaway a+b from both sides then I will get ab = -ba but is it ok to say that it is commutative just from that step or do I have to go further any more steps to prove it
I think for the part (ii) I have to do like as follows:
b(a+a) = 0;
therefore a+a =0 is it right
thx once again for ur help