The slope of a line that is perpendicular to the line in the graph is 0.
The slope of a line is a measure of its steepness, and it is calculated by dividing the change in y by the change in x. For a horizontal line, the change in y is 0, so the slope is 0.
The line in the graph is a horizontal line, so any line perpendicular to it will be a vertical line. Vertical lines have an infinite slope, but since the y-axis is labeled "Temperature (Fahrenheit)", and temperature cannot be infinite, the slope of the perpendicular line is simply 0.
Another way to think about it is that perpendicular lines intersect at right angles. A horizontal line intersects a vertical line at a right angle, so the slope of the vertical line must be 0.