To find the percentage difference between two numbers, we first need to find the absolute difference between them. In this case, the absolute difference between 51 and 63 is 12.

Next, we need to find the average of the two numbers. The average of 51 and 63 is (51+63)/2 = 57.

Now, we can find the percentage difference by dividing the absolute difference by the average and multiplying by 100. So, the percentage difference between 51 and 63 is (12/57) x 100 = 21.05%.

This means that 63 is 21.05% more than 51, or that 51 is 21.05% less than 63.