Explanation:
rotate the triangle in your mind (or as actual picture on your phone or computer), so that the right angle is the bottom right or bottom left, and C being the opposite bottom angle.
then we see
28 = cos(C) × 35
21 = sin(C) × 35
and so,
sin(C) = 21/35 = 3/5
cos(C) = 28/35 = 4/5
tan(C) = sin(C)/cos(C) = 3/5 / 4/5 = 3/4