I'm trying to do number 7 on page 10 of this document: http://www.edexcel.com/migrationdocu...athematics.pdf
I have done parts a and b. The answers give x=-4 but I don't know how that is obtained.
The method used for part b is questionable as I would use implicit differentiation. You cannot simply convert to an expression starting with y= when that type of equation isn't suitable. It is also questionable why the value of a is not used in b but that is a minor point.