Ahill that has a 33.9% grade is one that rises 33.9 m vertically for every 100.0m of distance in the horizontal direction. at what angle is such a hill inclined above the horizontal?

If you picture the hill and the flat ground under it as a right triangle,
you'll see that the 33.9 meters is the side opposite the angle, and the
100 meters of flat ground under it is the side adjacent to the angle.

(opposite) / (adjacent) is the tangent of the angle, so the angle is
whatever angle has a tangent of  (33.9/100) = 0.339.

That angle is about 18.73 degrees.  (according to our calculators, yours and mine)