Explanation

(a)
Let $P(3,\ - 4)$ be the foot of the perpendicular from the origin $O$ on the required line.
$Then,\ the\ slope\ of\ OP = \frac{- 4 - 0}{3 - 0} = \frac{- 4}{3}$
Therefore, the slope of the required line is $\frac{3}{4}$
$Hence,\ its\ equation\ is\ y + 4 = \frac{3}{4}(x - 3)$
$\Rightarrow \ 3x - 4y - 9 - 16 = 0$
$\Rightarrow \ 3x - 4y = 25$