Hey there!
When dealing with expressions, there are three terms that show up often: coefficient, variable, and constant. A variable is an unknown term commonly represented by a letter in equations (e.g. x, y, etc.) and a coefficient is a number attached to a variable (in your problem, 7 would be the coefficient of x, your variable, in 7x). That leaves your constant, which is simply a number that stays the same and doesn't contain a variable (e.g. 8, 10, 123, etc.).
Your answer will be "5".
Hope this helped you out! :-)