The difference between supplementary and complementary angles of any given acute angle is:

Let the given acute angle be $\theta$.The complementary angle of $\theta =90^{\circ }-\theta$.The supplementary angle of $\theta =180^{\circ }-\theta$.The difference between them is:
$$\text{Difference}=(180^{\circ }-\theta )-(90^{\circ }-\theta )=180^{\circ }-\theta -90^{\circ }+\theta =90^{\circ }$$