For any circle, the area A = πr² (where r is the radius)
The radius is always half the length of the diameter.
Since our diameter is 12, our radius must be 6.
Let's plug 6 in for r in our equation.
A = π(6)²
6² = 6×6 = 36
A = 36π in²
For π = 3.14...
A = 36×3.14 = 113.04 in²