This is probably the shortest proof.
Given m × 5 = m × 6,
[tex]5m=6m[/tex]
Subtract 2m from both sides,
[tex]3m=4m[/tex]
The equality didn't change just it's form since we are executing operations on both sides of the equation. Therefore,
[tex]5m=6m\Longleftrightarrow3m=4m[/tex]
Hope this helps.
r3t40