A crystal is mounted in a goniometer (an apparatus that allows it to be turned around its axis).
Using x-rays of wavelength 0.14 Angstroms, you observe maxima when the reading on the goniometer is 12.5 degrees, 14.6 degrees and 16.7 degrees. What is the spacing between the crystal planes? Note you do not know how the crystal is oriented in the goniometer.