If $x@y=xy-2x$, what is the value of $(5@3)-(3@5)$?
To solve this problem, I need to apply the given definition of the @ operation to both terms in the parentheses, and then simplify the expression. For the first term, I have $(5@3)=5\cdot 3-2\cdot 5=15-10=5$. For the second term, I have $(3@5)=3\cdot 5-2\cdot 3=15-6=9$. Therefore, $(5@3)-(3@5)=5-9=-4$. 