Hi can someone please help me with this:

In a triangle, ABC, AB=(2-x)cm, BC=(x+1)cm and angle ABC=120;

a) show that - i did this using the cosine rule.

b) Find the value of x for which AC has a minimum value: (I need to complete the square to find x)

I get this far;

Can you show me in stages how to resolve to find a minimum value for x?

Thanks,

F.