The total length is 49m^2
This is how I got the answer:
You have to set up the equation as so:
0.5 ( 3.14 * 7.5^2 - 3.14 * 5x^2 )
( * = multiply and ^ means squared )
Then after doing that it should look like this:
0.5 * 3.14 ( 56.25 - 25 )
Order of operations do what in the parentheses first
0.5 * 3.14 ( 31.25 )
When you multiply this out you get 49.087
Then you round up which gives you 49
Hope this Helps!