If reflected over y = x
then use
[tex]\left[\begin{array}{ccc}0&1\\1&0\end{array}\right][/tex]
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[tex]\left[\begin{array}{ccc}0&1\\1&0\end{array}\right] \left[\begin{array}{ccc}3&6&3\\-3&3&3\end{array}\right][/tex]
similar as (x, y) → (y, x)
multiply the matrixes to get the final answer
[tex]\left[\begin{array}{ccc}(3*0+1*-3)&(6*0+1*3)&(3*0+1*3)\\(3*1+(-3*0)&(1*6+0*3)&(1*3+0*3)\end{array}\right][/tex]
[tex]\left[\begin{array}{ccc}-3&3&3\\3&6&3\end{array}\right][/tex]
