Okay, so this equation is a percent error, and how you solve those is doing:
First Subtract Approximate value from Exact value:
4.50 - 4 = 0.5 , so the error is 0.5
Now, show the error as a percent of the exact value ... so divide by the exact value and make it a percentage:
0.5/4 = 0.125 = 12.5%
Therefore the percent error would be: 12.5%