2,4-(5,6-(1,75+(-2целых1/6+5целых1/

Раскрываем скобки постепенно изнутри:
-2,4-(5,6-(1,75-13/6+21/4))=-2,4-(5,6-(1,75-13/6+21/4))=-2,4-(5,6-(1,75+37/12))=-2,4-(5,6-1,75-37/12)=-2,4-5,6+1,75+37/12=37/12-6,25=
37/12-25/4=37/12-75/12=-38/12=-19/6