Two straight lines are drawn parallel to the straight line
\[
L\equiv 5x-12y-13=0
\]
which are at a distance of \(13\) units from the given line. Among these lines, if
\[
ax+by+c=0
\]
is the line which is closest to
\[
L=0,
\]
then
\[
\frac{a-2b+c}{a+b}
=
\]
Show Hint
If two parallel lines
\[
ax+by+c_1=0
\quad\text{and}\quad
ax+by+c_2=0
\]
are given, then the perpendicular distance between them is
\[
\boxed{
\frac{|c_2-c_1|}{\sqrt{a^2+b^2}}.
}
\]