During a series there are two teams, team A and team B. The probability for team A to win is and the probability for team B to win is . The winner of the series must win games. If i=4, find the probability that a total of 7 games are played. Also show that this probability is maximized when

so far I have:

now taking the derivative to find to max I get:

so my values would be

now it states that p is maximized at , can anyone help me figure out what I'm doing wrong.