because if you make x=18 than you multiply it times the other 18 then you get 324π cm², which is too much.
so if you subtract 2 from both the substituted x and the 18 to get 16² which is equal to 256, after that you multiply the leftover values which is 2 and 2 which equal 4
after that you simply add the 4 to 256 and you get 260π cm²
