Question

Write the slope of the line which is perpendicular to $y$-axis,

Answer 1

Line perpendicular to $y-$ axis is the $x-$ axis.
Inclination of $x-$ axis is $0^0$.
Hence, inclination of a line which is perpendicular to y-axis is $0^0$.

Slope of line with an inclination $\theta =\tan \theta$.

Hence, slope of line, with inclination $\theta ={90}^0$ is $\tan$ $0^0=0$.