Well, it seems tome that if he makes 91% of all his free throws, then every time he steps up to the line, there's a 91% chance that he'll make THAT throw.
Now the basketball has no memory, so the probability is the same for each attempt, and the probability of succeeding on two consecutive attempts is 91% of the probability of succeeding on the first one, and so on and so forth.
So the probability of 6 consecutive baskets is (91%) x (91%) x ... etc, 6 times.