What're you trying to find? By the looks of it, it appears to be Cos(θ) (the angle measure)
If that's the case, the formula for Cosine is: Adjacent/Hypothenuse.
Therefore, your equation would be this:
Cos(θ): 6/8.49.
However, because you're trying to find the ANGLE MEASURE and NOT the side- you gotta inverse it.
In which case, your equation would be:
Cos^-1 (8.49/6)
Plug that into a calculator: You get..
~54.75 in degrees.
