Answer: 
The percentage error is the absolute value of the real - experimental which is that all divided by the real and multiplied by 100%. 
11 meters
On the experimental is what he got, 12 meters
absolute error: 11 - 12 = |-1| = 1 
Round to the nearest 10th:
[(11 - 12)/11]*100 = |-9.09| = 9.09% = 9.10% 
Hope this helps