Concept:
The equations
\[
x^2-5x+6=0
\]
and
\[
y^2-6y+5=0
\]
represent pairs of parallel lines. Their intersections form the vertices of a parallelogram.
The diagonals join opposite vertices.
Step 1: Factorize the equations.
First equation:
\[
x^2-5x+6=0
\]
gives
\[
(x-2)(x-3)=0.
\]
Hence the pair of lines are
\[
x=2,\qquad x=3.
\]
Second equation:
\[
y^2-6y+5=0
\]
gives
\[
(y-1)(y-5)=0.
\]
Hence the pair of lines are
\[
y=1,\qquad y=5.
\]
Step 2: Find vertices of parallelogram.
The vertices are intersections:
\[
(2,1),\;(2,5),\;(3,1),\;(3,5).
\]
Step 3: Find equations of diagonals.
One diagonal joins \((2,1)\) and \((3,5)\).
Slope:
\[
m=\frac{5-1}{3-2}=4.
\]
Equation:
\[
y-1=4(x-2).
\]
Thus,
\[
y-1=4x-8
\]
which gives
\[
4x-y-7=0.
\]
Second diagonal joins \((2,5)\) and \((3,1)\).
Slope:
\[
m=\frac{1-5}{3-2}=-4.
\]
Equation:
\[
y-5=-4(x-2).
\]
Hence,
\[
y-5=-4x+8
\]
which simplifies to
\[
4x+y=13.
\]
Therefore the diagonals are
\[
\boxed{4x-y-7=0,\;4x+y=13}.
\]