Combine like terms: add all the coefficients of \(x\).<br><br>
&bull; \(x\) means \(1x\)<br>
&bull; \(x + x + 3x = 1x + 1x + 3x\)<br>
&bull; \(1 + 1 + 3 = 5\), so the result is \(5x\)<br><br>
Final Answer: C: \(5x\)