Answer:
Step-by-step explanation:
follow the Pemdas order of operations
add and subtract (left to right)
apply the rule +(-a)=-a
+(-78)=-78
=54-78-(-39)
54-78=-24
=-24-(-39)
apply the rule -(-a)=+a
-(-39) =+39
=-24 +39
-24+39=15
=15
