34.5×76.5-345×6.42-12.3×14.5
=34.5×76.5-34.5×64.2-12.3×14.5
=34.5×(76.5-64.2)-12.3×14.5
=34.5×12.3-12.3×14.5
=(34.5-14.5)×12.3
=20×12.3
=246