Choose the correct ordered pair

It is B.

x+y=3
y=9

To solve the equation you would substitute 9 in for y (in the first equation) and get
x+9=3
You would then subtract 9 from both sides of the equation to get
x=-6
B is the only choice where -6 is first in the ordered pair, but just to make sure, you could substitute -6 back in to the equation to get
-6+y=3
Add 6 to both sides to get y=9, and you know that the correct ordered pair is 
(-6,9)

First, let's plug the information we have into the equation. 
x + 9 = 3

Next, you would have to do is isolate the x. To do this, you would subtract both sides by 9. 
x + 9 = 3
x = -6

So, this is what we know. 
y = 9
x = -6

Now, all you would have to do is change them into coordinates. (x,y)
(-6,9)

Your answer would be B. (-6,9). 

I hope this helps!