brainly won't let me type my answer out so i'll just put it in the comments, sorry if it's inconvenient.
The diagram of the lawn and the shed is shown below.
The area of the lawn needed to be mowed equals to the area of the yard minus the area of the shed
The area of the yard =

The area of the shed =

The area of the lawn =

The area of the lawn =

The area of the lawn =
We need the exponents law

So if
a=3
b=1/2
then

where x represent every half a year.