So i have this question: "Consider two independent tosses of a biased coin, which lands heads up with probability 2/3. Suppose you win/loose $1 whenever the coins lands heads/tails up. Let X1 and X2 denote your cumulative winnings after 1 respectively 2 tosses. - CALCULATE the conditional expectation E(X1|X2)"

I have tried to make X1 & X2 table, then find out f (X1|x2), and then the expectation.. but lack of routine with this kind of calculations makes me confused. Thank you.