Consider a pyramid OPQRS located in the first octant with O as origin, and OP and OR along the x-axis and y-axis respectively. The base OPQR is of the pyramid is a square with OP = 3. The point S is directly above the mid point T of the diagonal OQ such that TS = 3. The equation of the plane containing the triangle OQS, is

x - y = 0 

We are given three points (0, 0, 0), (3, 3, 0), (3/2, 3/2, 0)

so plane passing through these

$⇒\begin{vmatrix}x-0&y-0&z-0\\3-0&3-0&0-0\\3/2-0&3/2-0&0-0\end{vmatrix}=0$

$⇒x-y=0$