An arc only exists on the outside, or the circumference of a circle. To find the length of this arc, we need to find the part of the circumference which this arc covers. The part is given in the problem: 45 out of 360 degrees.
Circumference = 2 x radius x pi
Circumference = 2 x 18 x pi
Circumference = 36pi
Now, we only need 45/360 or 1/8 of the total circumference.
1/8 of 36pi = 9pi/2 or 4.5 pi
Answer: 9pi / 2 or 4 1/2 pi or 4.5pi cm
Hope this helps!