The expression is 

               (Substitute x = 10, y = – 4 and z = – 15 in original expression)

Additive inverse of  4 is – 4. To subtract – 4, add the additive inverse of 4.

                  (Add: )

Additive inverse of – 15 is 15. To subtract –15, add the additive inverse of –15.

                  (Add: )

The difference is 21.