-11 -(-12) - (-15)

$${\mathtt{\,-\,}}{\mathtt{11}}{\mathtt{\,\small\textbf+\,}}\left({\mathtt{12}}\right){\mathtt{\,\small\textbf+\,}}\left({\mathtt{15}}\right) = {\mathtt{16}}$$

-11-(-12)-(-15)= -11+12+15= 16