1) x+y= -3

x + y + 3 = - 3 + 3

x + y + 3 = 0

2) 6x + y = 0

6x + y - 6x =0 - 6x

y = 0 - 6x

y = - 6x

3) 2x - 3y = 9

2x - 3y - 9 = 9 - 9

2x - 3y - 9 = 0