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
