I knw that the P/C parity is: C + B/(1+RFR) = P + S however I came across another formula which is: C + [X * e^(-R*T)] = P + S … How does [X * e^(-R*T)] = B/(1+RFR) ?

(X*e^(-R*T)) is the continuous compounding form of X/(1 + RFR).

anytime you see e^ something it is continuous compounding.