18.
A line perpendicular to the x-axis is parallel to the y-axis.
That means it's a vertical line.
All points of a vertical line have the same x-coordinate.
For example, if a vertical line passes through point (1, 0), then the equation of the line is x = 1. All points on that line have x-coordinate 1, no matter what the y-coordinate is.
In this case, the vertical line passes through point (-10, 5), so the equation of the line is
x = -10.
Answer: A. x = -10
19.
f(x) = 2/3 x - 1
They want to know f(9).
Just replace x with 9 and evaluate the expression.
f(9) = 2/3 * 9 - 1
f(9) = 18/3 - 1
f(9) = 6 - 1
f(9) = 5
Answer: f(9) = 5
20.
