It is important because if you are adding and subtracfing and the decimals are off when your done with the equation you might end up with two decimals or numbers in the wrong place
Answer:

Step-by-step explanation:
This is just a perfect square
Answer:
-2y + 3 = 0
Step-by-step explanation:
x - 6y - 9 = 0
-6y - 9 = -x subtract x from both sides
6y + 9 = x divide both sides by -1
x = 8y + 6
6y + 9 = 8y + 6 replace x with 6y + 9
-2y + 9 = 6 subtract 8y from both sides
-2y + 3 = 0 subtract 6 from both sides