Verify the identity
cot x - tan x = sec x(csc x - 2 sin x)

Work Shown:

You'll need to use a trig ratio. Specifically, we'll be using cosine here

cos(angle) = adjacent/hypotenuse

cos(x) = 13/20

cos(x) = 0.65

x = arccos(0.65)

x = 49.4583981264954 which is approximate

x = 49

The missing angle is approximately 49 degrees

Make sure your calculator is in degree mode. The notation arccos refers to arccosine and it's the same as inverse sine.