Find the Cartesian equation of the plane $\;(a)\; \overrightarrow r. (\hat i + \hat j - \hat k) = 2$

Question

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$\begin{array}{1 1} x+y-z=2 \\ x+y+z=-2 \\ x+y+z=2 \\ x+y-z=-2 \end{array}$