You might note that .
And so .
That may not be any better than what you have done.
I'm not sure how to deal with this problem (see attachment) when they have a treated as a real number. I know I have to use the modulus theorem but I think it may just be the algebra involved with leaving a as a real number that is giving me the problem. I have attached my working if it's not clear what I mean. (see attachment)