18) `x^2 + 9y^2 + 6y + 5 + 4x`
`= (x^2 + 4x + 2^2) + (9y^2 + 6y + 1)`
`= (x+2)^2 + (3y + 1)^2`
20) `x^2 + 20 + 9y^2 + 8x - 12y`
`= (x^2 + 8x + 4^2) + (9y^2 - 12y + 2^2)`
`= (x + 4)^2 + (3y - 2)^2`
22) `4y^2 + 34 - 10x + 12y + x^2`
`= (x^2 - 10x + 5^2) + (4y^2 + 12y + 3^2)`
`= (x - 5)^2 + (2y + 3)^2`
24) `x^2 + 9y^2 - 12y + 29 - 10x`
`= (x^2 - 10x + 5^2) + (9y^2 - 12y + 2^2)`
`= (x - 5)^2 + (3y - 2)^2`
26) `4y^2 - 12x + 12y + 9x^2 + 13`
`= (9x^2 - 12x + 2^2) + (4y^2 + 12y + 3^2)`
`= (3x - 2)^2 + (2y + 3)^2`
28) `x^2 + 17 + 4y^2 + 8x + 4y`
`= (x^2 + 8x + 4^2) + (4y^2 + 4y + 1)`
`= (x + 4)^2 + (2y + 1)^2`
30) `x^2 + 20 + 9y^2 + 8x - 12y`
`= (x^2 + 8x + 4^2) + (9y^2 - 12y + 2^2)`
`= (x + 4)^2 + (3y - 2)^2`
