Explanation

(a)
The equation of given line is
$y = mx + \frac{a}{m}\ \ \ldots(i)$
The equation of line perpendicular to Eq. (i) is
$my + x + \lambda = 0\ \ \ldots(ii)$
This line passing through $(a,\ 0)$.
$0 + a + \lambda = 0\ \Rightarrow \ \lambda = - a$
On putting this value on $\lambda$ in Eq. (ii) and solving with Eq. (i), we get
$x = 0\ and\ y = \frac{a}{m}$
Coordinates of the foot of perpendicular are $\left( 0,\frac{a}{m} \right).$