4(7/12) - 7(-5/7)

=28/12 + 35/7

= 14/6 + 35/7

= 49/6 + 35/7

= (497 + 356) / (6*7)

= (343 + 210) / 42

= 553 / 42

= 23/3

= 7 2/3

Поэтому, 4x - 7y при x=7/12 и y=-5/7 равно 7 2/3.