You can put it in a calculator, or do long division on a piece of paper. I would show you how to break it down step-by-step, but doing long division on here is complicated.
All you have to do is find the decimal of a fraction is to divide the numerator and the denominator.
In this case, divide 37 by 40 to get your decimal.
This is because the diameter of the smaller circle is 12pi/pi which is 12, and the diameter of the bigger one is 18pi/pi which is 18. 12 + 6 = 18, so the diameter increases by 6.