(Make sure you are in degrees instead of radians.)
We are given opposite and hypotenuse.
Use Soh-Cah-Toa:
\(\sin\theta=\frac{opp}{hyp}\)
\(\rightarrow\sin\theta=\frac{3.4}{5.8}\)
\(\rightarrow\theta=\sin^{-1}\frac{3.4}{5.8}\)
In degrees, this is about 35.9°
