I have a question!!

book's question is what is P(Inf|ELISA+,WB+)

then. I think P(Inf | ELISA+, WB+) ={P(ELISA+|Inf, WB+)P(Inf|WB+)} / {P(ELISA+|Inf,WB+)P(Inf|WB+)+P(ELISA+|NotInf,WB+)P(|NotInf|WB+)}

I can't understand step 3/4. please explain!!