21: Coefficients are *constants* – numbers like 3, 4/5, the square root of 2. A *variable* can change. For instance, in the equation y = 4x, the term 4x has the coefficient 4. We can’t change that, but we *can* change the x to whatever value we’d like to obtain different values for y.
22: 4x + 7 is an *expression*. In order to make it an equation, we’d have to, well, set it equal to something! All equations require something to be equal to something else, so a statement like 4x + 7 = 23 would be an equation, but 4x + 7 on its own is not.