Vector Algebra

Let O be the origin. Let $\overrightarrow {OP} = x\widehat i + y\widehat j - \widehat k$ and $\overrightarrow {OQ} = - \widehat i + 2\widehat j + 3x\widehat k$, x, y$\in$R, x > 0, be such that $\left| {\overrightarrow {PQ} } \right| = \sqrt {20}$ and the vector $\overrightarrow {OP}$ is perpendicular $\overrightarrow {OQ}$. If $\overrightarrow {OR}$ = $3\widehat i + z\widehat j - 7\widehat k$, z$\in$R, is coplanar with $\overrightarrow {OP}$ and $\overrightarrow {OQ}$, then the value of x² + y² + z² is equal to :