The diagram of the parking lot is:
We can draw a line to divide the figure into a rectangle and a right triangle:
And we can see that:
BD = EC
AE = AB - CD
And since we know:
AB = 21m
CD = 15m
We can find the length of AE:
Now, we have a right triangle:
We can find the length of EC using the pythagorean theorem:
Then, we can substitute with the known values:
And solve for EC:
Thus:
But, we need to write the answer in cm. Since there are 100 cm in a meter:
The answer is: