I believe you are trying to make the words into an actual problem so it's visible. Sum is adding, and a number would be a variable like x or n.
The answer to your question would be 42+n = 51. To simplify 42+n=51 we must subtract 42 from both sides, (42-42 and 51-42). Now that 42 has been eliminated we subtract 42 from 51 which gives you 9, your simplified answer is n=9.
