Given that the certain area is 18km in a circle
the whole circle is 360 .out of it other part has 225 so the 18km part has the angle about 135 degrees.
so,18km=135 -->1
x=225 -->2
on cross multiplying 1 and 2 we get
18x225=135x
4050=135x
x=4050/135
x=30
so the answer is 30km