How to find a angle measure with the pythagorean theorem?

The Pythagorean Theorem states that: "the sum of the squares of the legs is equal to the square of the hypotenuse." 
a² + b² = c² or  c² = a² + b²

Note¹: Hypotenuse is the side opposite the right angle.
Note²: The legs are the sides that are not opposite the right angle.

Examples: (Use the attachments for better understanding:)

*First example






Second example: