Help with this A2 maths question

I'm having a lot of difficulties solving question 6(i) of this exam. The mark scheme is no help at all... I don't even know whether I'm going in the right direction or not. Can anyone please give me a hand?

This is the exam:
https://docs.google.com/viewerng/viewer?url=http://maxpapers.com/wp-content/uploads/2012/11/9709_s14_qp_32.pdf
This is the mark scheme:
https://docs.google.com/viewerng/viewer?url=http://maxpapers.com/wp-content/uploads/2012/11/9709_s14_ms_32.pdf

And this is my working:


Thanks in advance

A is the centre for the arc BC so AB=AC=r Does that help ?

Let AB=AC=R then $R=2r\cos x\text{and }2R+2xR=\pi R$
Can you finish it from here ?

But why is R=2rcos x? Which formula are you using

Questions?

AB is a chord of a circle. How do you find the length of a chord ?