The sum of negative eighteen and a number is eleven. What is the number?

29

Step-by-step explanation:

-18 (Negative Eighteen) = A

29 (Positive Twenty Nine) = B

11 (Positive Eleven) = C

First, make A positive, and add C to it. You get B.

Now here is the equation:

-18 + (18 + 11)

-18 + (29)

11