I order to calculate an angle you'll need at least three points or a reference system (e.g. relative to sprite x on an unit circle).
Either way it's a mix of vector math and trigonometry and that's as basic as it gets.
For SFML 3, we've actually added one option to the
sf::Vector2<T> class, so this then might already make things slightly easier.
You can't really avoid learning some of vector math and trig and learn how to apply it.
If you enter for example "angle between two vectors" into a web search engine, you'll find some good explanations.
