What is the slope of the line x = - 5?

X = - 5 is a line parallel to y-axis and has no y-intercept, then the slope in undefined.

Depending on your grade, you can say the slope = tan (Ф) and in this case angle Ф = 90° and tan (90°) = undefined (or in some book →∞)

X = - 5 is a vertical line and therefore has an undefined slope.