Consider x=y=1 and z = 3.
Then \((x \oslash y) \oslash z = (1 \oslash 1) \oslash 3 = 4 \oslash 3 = 14\).
But \(x \oslash (y \oslash z) = 1 \oslash (1 \oslash 3) = 1 \oslash 8 = 18\).
So \((x \oslash y) \oslash z \not = x \oslash (y \oslash z)\).
Did you give specific concrete values for x, y, and z?
Did you briefly explain why your values were a counter-example to the claim?