Câu 1: Cho $$\vec{a}(1; -1; 3)$$, $$\vec{b}(2; 0; 1)$$, $$\vec{c}(x; 2; -3)$$. Tìm $$x$$ để $$\vec{a}, \vec{b}, \vec{c}$$ đồng phẳng.