I used SUM() to avoid errors whenever there are empty cells in columns C or D.
In case you want the formula to return nothing, when one component is empty, then
Code:
=IFERROR($C11 + $D11, "")
EDIT:
I looked again at your Word image. Do you really want to be the formula result be bigger than Current Budget in case Adjusted Budget is negative? Very strange logic, but whatever!
Code:
=SUM($C11) + ABS(SUM($D11))