Because it states that quadrilateral ABCD is a rectangle (sorry, forgot to say that in my image), that means that angles ∠ABC, ∠BCD, ∠CDA and ∠DAB are all right angles. That also means that all those angles are 90°.
Since ∠ACD is a portion of ∠BCD, (37° is a portion of 90°), and we're looking for the complement of ∠ACD, all we have to do is subtract 37° from 90°.