The percentage error is calculated by taking the absolute difference between the measured value and the true value, dividing it by the true value, and then multiplying by 100 to express it as a percentage. 

In this case, the true value is 15 cm and the measured value is 16.8 cm. So, the percentage error is:

\(\frac{|16.8 - 15|}{15} \times 100\% = \frac{1.8}{15} \times 100\% \approx 12\%\)