2.15: Line parallel to $AB$ through $C$; length $DE$

Find the equation of the line, $L$, that is parallel to the line $AB$ and passes through $C$. Give your answer in the form $ax + by = c$, where $a$, $b$, and $c$ are integers.

The line $L$ meets the $x$-axis at the point $D$ and the $y$-axis at the point $E$. Find the length of $DE$.