I suppose you want to solve this for x.
The sum of 16 and x is [tex]16+x[/tex]
We want this sum to be -11, so we want
[tex]16+x=-11[/tex]
When dealing with an equation, we can add the same number to both sides: if we add -16 to both sides (i.e. if we subtract 16 from both sides) we will solve for x:
[tex]16+x-16=-11-16 \iff x=-27[/tex]