To simplify the expression 3x + 7, we look for like terms that can be combined. In this case, the term 3x is a variable term, while 7 is a constant. Since these two terms are not alike, they cannot be combined further.
Therefore, the expression 3x + 7 is already in its simplest form. It represents a linear expression where 3x indicates that the value of x is multiplied by 3, and then 7 is added to that product.
In conclusion, the simplified form of the expression is simply 3x + 7.