Chapter F - Foundations: A Prelude to Functions - Section F.3 Lines - F.3 Assess Your Understanding - Page 30: 58

$x=4$

The equation of a vertical line is always in the form of: $x=a$, where $a$ is constant. Here, the point given is $(4,-5)$, which is on the line. For this point, $x=4$ and this should be true for the other points on the line. The equation is, therefore: $x=4$