To simplify the expression \( 4x + 8x \), you can apply the properties of operations, specifically the distributive property and the combining like terms.
Here, \( 4x \) and \( 8x \) are like terms because they both contain the variable \( x \). You can combine them:
\[ 4x + 8x = (4 + 8)x = 12x. \]
Thus, the simplified expression is:
12x.