Okay now the answer Mastering accepted was 0.0010 cm, if you actually calculate it which is the same as your 1.0 E^-5 except that answer was in m not centimeters, so you multiply it by 100cm/m.
You might want to check to make sure you used the right alpha levels (23 E^-6 for aluminum and 12 E^-6 for steel)
Hope this helps!
