[tex]A(-2,a);\quad B(2,2);\quad C(-1,-7).\\ \\ A,B,C \text{ coliniare}: \\ \\ \begin{array}{rcl} \Rightarrow &&m_{AB} = m_{BC} \\ \\ && \dfrac{y_B - y_A}{x_B-x_A} = \dfrac{y_{C} - y_{B}}{x_C - x_B} \\ \\&& \dfrac{2-a}{2-(-2)} =\dfrac{-7-2}{-1-2} \\ \\ && \dfrac{2-a}{4} = \dfrac{-9}{-3} \\ \\ && 2-a = 4\cdot 3 \\ \\ &&a = 2 - 12 \\ \\ && \boxed{a = -10}\end{array}\\ \\ [/tex]
