Hello from MrBillDoesMath!
Answer: A line with slope -3 with y intercept 9
Discussion:
Subtracting 3x from both sides of
3x + y = 9
gives
3x - 3x + y = 9 -3x
or
y = 9 -3x
This is a line (has the form y = mx + b where m = -3 (the slope) and b = 9) As the slope is negative, the line is descending or pointing "downward" as x increases.
Regards, MrB