1. The problem is to simplify the expression $3x + 4x$. 2. We use the rule that like terms can be added by adding their coefficients. Here, both terms have the variable $x$. 3. Add the coefficients: $$3 + 4 = 7$$ 4. So, $$3x + 4x = 7x$$ 5. The simplified expression is $7x$. Therefore, the correct answer is option b) $7x$.