To solve the given expressions, we just have to add and subtract. Remember that the different signs mean we have to subtract and the same signs mean we have to add.
[tex]\begin{gathered} 56+(-56)=56-56=0 \\ -240+370=130 \\ -5.7+(-4.2)=-5.7-4.2=-9.9 \\ -15+(-8)=-15-8=-23 \end{gathered}[/tex]