Simplify the given algebraic expression. \[ 6 x+9 x \] \[ 6 x+9 x= \]

Step 1 ：The given expression is \(6x + 9x\).

Step 2 ：These are similar terms, meaning they have the same variable part.

Step 3 ：In such cases, we can simply add the coefficients of the similar terms to simplify the expression.

Step 4 ：The coefficient of the first term is 6 and the coefficient of the second term is 9.

Step 5 ：Adding these coefficients together gives us 15.

Step 6 ：Therefore, the simplified form of the given expression is \(15x\).

Step 7 ：Final Answer: \(\boxed{15x}\)