18. A line perpendicular to the x-axis is a vertical line.
The equation of a vertical line is
x = k,
where k is the x-coordinate of every point on the line.
Since this vertical line goes through point (-10, 0.5), and this point has x-coordinate -10, then every point on the line has x-coordinate -10.
The equation is
x = -10
19.
To find f(9), just replace x with 9 and evaluate the expression.
20.
