This is LaTeX code:
[latex]
P( \{a, b\} \cup \{b,c\}) = P( \{a, b\} ) + P( \{b,c\}) - P( \{a, b\} \cap \{b,c\}) = P( \{a, b\} ) + P( \{b,c\}) - P( \{b\})
[/latex]