a) (+8)-(-7) b) (-57.2)+(-42.8) c) -3+5-(-4-9) d)12-(3-6)-15

a) 15
b) -100
c) 15
d) 0

Bonjour  Laraa01

[tex]a)\ (+8)-(-7)=+8+7\\(+8)-(-7)=8+7\\\boxed{(+8)-(-7)=15}\\\\ b) (-57,2)+(-42,8)=-57,2-42,8\\\boxed{(-57,2)+(-42,8)=-100}\\\\c)\ -3+5-(-4-9)=-3+5-(-13)\\-3+5-(-4-9)=-3+5+13\\-3+5-(-4-9)-3+18\\\boxed{-3+5-(-4-9)=15}[/tex]

[tex]\\\\d)\ 12-(3-6)-15=12-(-3)-15\\12-(3-6)-15=12+3-15\\12-(3-6)-15=15-15\\\boxed{12-(3-6)-15=0}[/tex]