11. We define: $a @ b=a \times(a+1) \times \ldots \times(a+b-1)$. Given that $x @ y @ 2=420$, then $y @ x=$ ( Exam point: Solving equations and defining new operations
【Analysis】Let $\mathrm{x} @ \mathrm{y}=\mathrm{m}$, then $\mathrm{m} @ 2=420=\mathrm{m}(\times m+1 \Rightarrow) m=20$
That is
$x @ y=20=4 \times 5 \Rightarrow x=4, y=2$
$x @ y=20=1 \times 20=4 \times 5$
$x=20, y=1$ or $x=4, y=2$
Then $y @ x=1 @ 20=20$ !; or $y @ x=2 @ 4=2 \times 3 \times 4 \times 5=120$
Answer: 20 ! or 120