When two angles are complementary, it means that the two angles add up to 90°. Thus when angle a is 72° when angle a and angle b are complementary,
angle a + angle b = 90°
72° + angle b = 90°
*minus 72° on both sides of the eqn*
angle b= 90°-72°
angle b= 18°
I hope this helps! Feel free to clarify if you have any questions!