Let's start by thinking about what we know about a circle.
1) Circumference C = 2(Pi)D
2) Total Angle in a circle is 360 degrees
Since they gave you theta, just figure out what fraction of the total angle of the circle this is: 150/360 = .4167
Then calculate the Total Circumference of the Circle = 2(3.14)(13.9) = 87.3
So the arc length is (.4167) * (87.3) = 36.38 which rounds to 36.4
Does that make sense? If not post a follow-up comment and we'll go from there.