Come on, this is pretty easy! Let's assume that one player uses a constant amount of balls in a constant time. If 9 players use 30 balls in 2 days, it means that one player uses $$ \frac{30}{9\times 2} = \frac{5}{3} $$ per one day. Then 11 players would use $\frac{5}{3}\times 11 \times 3 = 55$ tennis balls in three days.