Hello, Jasmine,
Let's put the sentence "17 is the sum of a number and 6." into a formula:
17 is the sum of a number and 6
17 = X + 6
17=X+6
Subtract 6 from both sides to isolate X:
11 = X
The vales of X is 11. Now we can add the number(11) to 6, and we'll get 17, jsut as the statement promises.
I hope this helps,
Bob