In constructing a plane equation V in 3D, we should know three points, for example, $P(x_1, y_1, z_1)$, $Q(x_2, y_2, z_2)$, and $R(x_3, y_3, z_3)$.
Why for arbitrary point $X$ in $V$, vector $PX = \lambda (PQ) + \mu (PR)$, where $\lambda$ and $\mu$ are real numbers?.
as far as I know, not every vector can be a linear combination of other vectors