Your question is very vague so I do hope this answer is sufficient.

When you have an unknown quantity, you can call it x,y or any letter actually although x is the most common.

- Let the unknown number `=x`

Next consider what you are required to do. We are looking for "the sum of" ...which mean we must ADD our unknown to 7

- `x+7`

So this is now complete. The answer to your question is: **x+7**