so.. hmm notice the picture below
segments AB and AC are perpendicular, thus, the angle atop on the left-side is 90 - 56 then
now, on the triangle on the left-side, you have two angles, recall, all internal angles in a triangle add up to 180, so 180 - [ those two angles] is ADB
now, the angle ADB lies on the flat-line or segment of BC, is a flat-line, so the angle on the other side of the AB line, is 180 - ADB, because ADB + that angle = 180
and to get ACB, well, by then you'd have two angles on the right-side triangle, the 56 one, and the one next to ADB, the supplementary one... well, 180 - those two angles is ACB