27. Given that $a, b, c$ are two non-negative numbers, and satisfy $3a + 2b + c = 5, 2a + b - 3c = 1$, let $m = 3a + b - 7c$, $x$ be the maximum value of $m$, $y$ be the minimum value of $m$, then the value of $xy$ is $\qquad$
Answer: $\frac{5}{77}$