WAEC Further Mathematics 2014 Paper

When you have two lines, $y_{1}, y_{2}$, perpendicular to each other, the product of their slopes = -1.

$3x + 4y + 6 = 0 \implies 4y = -6 - 3x$

$\therefore y = \frac{-6}{4} - \frac{3}{4}x$

$\frac{dy}{dx} = \frac{-3}{4}$

Also, $4x - by + 3 = 0 \implies by = 4x + 3$

$y = \frac{4}{b}x + \frac{3}{b}$

$\frac{dy}{dx} = \frac{4}{b}$

$\frac{-3}{4} \times \frac{4}{b} = -1 \implies \frac{4}{b} = \frac{4}{3}$

$b = 3$