3x²+3y²-12x-6y-30=0

Answer 1

first, get rid of all those multiples of 3.
x^2+y^2-4x-2y = 10
x^2-4x+4 + y^2-2y+1 = 10+4+1
(x-2)^2 + (y-1)^2 = 15