Answer:
1
Explanation:
You want the slope of the line perpendicular to y = 9 -x.
Slope
The slope of the given line is the coefficient of x: -1.
Perpendicular
Lines are perpendicular when one has a slope that is the opposite of the reciprocal of the slope of the other. The slope of the desired line is ...
m = -1/(-1) = 1
Any line perpendicular to y = 9 -x will have a slope of 1.