The circle shown has the center o and the measure of angle aob is 45. if the area of the shaded sector is 18 pi what is the radius of the circle

The picture in the attached figure

we know that
area of the circle=pi*r²

if 45° has an area-------> 18*pi units²
360° (full circle)----------> x
x=360*18*pi/45-----> x=144*pi units²

area of circle=pi*r²
144*pi=pi*r²------> r²=144--------> r=√144-----> r=12 units

Where are the following equations?