(a) Given,
BD : DC = 2 : 1.
Let coordinates of D be (x, y)
By section-formula,
(x, y) = $\Big(\frac{m_1x_2 + m_2x_1}{m_1 + m_2}, \frac{m_1y_2 + m_2y_1}{m_1 + m_2}\Big)$
Substituting values, we get :
$$\begin{aligned}\Rightarrow (x, y) = \Big(\frac{2 \times 8 + 1 \times (-4)}{2 + 1}, \frac{2 \times -6 + 1 \times 3}{2 + 1}\Big) \\[1em] \Rightarrow (x, y) = \Big(\frac{16 - 4}{3}, \frac{-12 + 3}{3}\Big) \\[1em] \Rightarrow (x, y) = \Big(\frac{12}{3}, \frac{-9}{3}\Big) \\[1em] \Rightarrow (x, y) = (4, -3).\end{aligned}$$
Hence, coordinates of D = (4, -3).
(b) By mid-point formula,
Mid-point = $\Big(\frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2}\Big)$
Given,
M(6, 0) is the mid-point of AD.
$$\begin{aligned}\therefore (6, 0) = \Big(\frac{a + 4}{2}, \frac{b + (-3)}{2}\Big) \\[1em] \Rightarrow (6, 0) = \Big(\frac{a + 4}{2}, \frac{b - 3}{2}\Big) \\[1em] \Rightarrow \frac{a + 4}{2} = 6 \text{ and } \frac{b - 3}{2} = 0 \\[1em] \Rightarrow a + 4 = 12 \text{ and } b - 3 = 0 \\[1em] \Rightarrow a = 12 - 4 = 8 \text{ and } b = 3.\end{aligned}$$
A = (a, b) = (8, 3).
Hence, coordinates of A = (8, 3).
(c) By formula,
Slope of line = $\frac{y_2 - y_1}{x_2 - x_1}$
Slope of line BC = $\frac{-6 - 3}{8 - (-4)} = \frac{-9}{12} = -\frac{3}{4}$.
We know that,
Slope of parallel lines are equal.
Slope of line parallel to BC = $-\frac{3}{4}$.
By point-slope form,
Equation of line : y - y1 = m(x - x1)
Equation of line parallel to BC and passing through M is :
⇒ y - 0 = $-\frac{3}{4}(x - 6)$
⇒ y = $-\frac{3}{4}(x - 6)$
⇒ 4y = -3(x - 6)
⇒ 4y = -3x + 18
⇒ 3x + 4y = 18.
Hence, equation of the required line is 3x + 4y = 18.