I'm Cikadamate, I joined just because I am a year ahead in maths but I'm struggling. I am hoping that when necessary I will be able to use this forum to help me understand anything I am stuck on.

Right now we're doing co-ordinate geometry and This is the question which prompted me to join:

A circle has centre G(2,-1). A point H(a,3) on the circumference of the circle is joined to the centre of the circle. if the radius of the circle is 8, find 'a'.

What I need to know is the method for working this type of question out.

Any help would be most appreciated