Method A.
A full circle has a central angle of 360 degrees.
A central angle of 90 degrees is a quarter of a circle.
The area of the sector is 1/4 of the area of the circle.
Find the area of the circle and divide by 4.
Method B.
You can also use the formula for the area of a sector with a central angle of n degrees.
[tex] A_{sector} = \dfrac{n}{360^\circ} \pi r^2 [/tex]
[tex] A_{sector} = \dfrac{90^\circ}{360^\circ} \times 3.14159 \times (12~cm)^2 [/tex]
[tex] A_{sector} = 113.1 ~cm^2 [/tex]
