For the first one, is not equal to .
You should start by squaring both sides.
b) Your working has been marked incorrect because you have failed to realise that multiplying or dividing both sides of an inequality changes the inequality sign. It's quite possible that your denominators could be negative. The easiest way to do this is first to note that and , then...
Now to solve this, you need to consider two cases, the first where and the second where .
Case 1:
which gives
Case 2:
which gives
So the solution is .