Circles and triangles

Gather the x terms and y terms together and try your best to complete the square. Try to set your equation out like this:
(x+a)^2 + (y+b)^2=r^2
Where a and b are numbers
From this, you can find the coordinates
I like to make (x+a)=0 to find x, which will be your x coordinate. Do the same for (y+b) to find the y coordinate
These coordinates will be the centre of your circle

thank you this is what i did for part a. its just part c i cant get my head around!!

Hint: What's the diameter of the circle?