Question

If the points(-3, 6), (-9, a) and (0, 15) are colliner then find a

Answer 1

Let the given points be A(-3, 6), B(-9, a) and C(0, 15)
Since the points A, B and C are collinear the slope of $\overleftrightarrow{AB}$ = the slope of $\overleftrightarrow{BC}$
$\Rightarrow \frac{6-a}{6}=\frac{15-a}{9}$
$\Rightarrow$ $18-3a=30-2a\Rightarrow -12$