Finding missing coordinates for 3x + y = 9

[georgebaseball]

How can I find the missing coordinate for points on a line? The line is:

. . .3x + y = 9

The points are:

. . .a) (3, ?)
. . .b) (?, -6)

This is how I've been trying to solve (a):

First I replace, so:

. . .3(3) + y = 9
. . .9 + y = 9

Then I solve:

. . .y = 9 - 9 = 0

For (b), I do the same thing:

. . .3x + (-6) = 9
. . .3x - 6 = 9
. . .3x = 3
. . .x = 1

What am I doing wrong?

Thank you for your help!

 

[wantedcriminal03]

3x - 6 = 9
3x = 3
x =1

what am i doing wrong?

Erm...

3x - 6 = 9

Add 6 to both sides

3x = 15
x = 15/3 = 5