Two vectors $\vec{x}$ and $\vec{y}$ are collinear if $\vec{x}$ = $\lambda\vec{y}$ , where λ is a constant Now, the vectors $\hat{2i} - \hat{3j} + \hat{4k}$ and $\hat{ai} + \hat{6j} - \hat{8k}$ are collinear ∴ $\hat{2i} - \hat{3j} + \hat{4k}$ = λ . $(\hat{ai} + \hat{6j} - \hat{8k})$ , where λ is a constant. ⇒ 2 = λ a , - 3 = 6 λ , 4 = - 8 λ Now, - 3 = 6λ or 4 = - 8λ ⇒ λ = $\frac{-1}{2}$ 2 = λa ⇒ 2 = $\frac{-1}{2}$ a ⇒ a = - 4