1. The problem is to understand how to draw a straight vertical line on a graph. 2. A vertical line has the same $x$-coordinate for all points on the line. 3. The equation of a vertical line is $x = a$, where $a$ is a constant number. 4. This means no matter what the $y$-value is, $x$ remains fixed at $a$. 5. For example, the line $x = 3$ is vertical and passes through all points where $x$ is 3. 6. To draw it, plot all points with $x = a$ and varying $y$ values. 7. This line does not have a function form $y = f(x)$ because it fails the vertical line test. 8. So, the number that gives a vertical line is the constant $x$ value in the equation $x = a$.