I have the following conic and i want to find its centre of symmetry: 3y^2+2x+18y=-31

I tried the following way but it didn't seem to work, could somebody tell me where i went wrong?

(3y^2+18y)+2x=-31

3(y^2+6y)+2x=-31

completing the square:

3(y^2+6y+9)+2x=-31+(3*9)

3(y^2+6y+9)+2x=-4

3(y+3)^2+2x=-4

(3(y+3)^2)/(-4)+2x/(-4)=1

so the centre is (0,-3)?

Where am i going wrong?

Thanks