The starting solution using northwest$-$corner method is:

$($Note: You must enter a shipment in every cell, even if the shipment is $0.)$

$\backslash$table$[[\backslash$table$[[$From$],[$Mexico City$]],$To$,\backslash$table$[[$Supply$],[120]]],[$Los Angeles,Calgary,Panama City$],[,$$$6,,$$$18,,$$$8$