Answer
See below
Work Step by Step
6 feet = 72 inches
$\text{z-score}=\frac{\text{data item-mean}}{\text{standard deviation}}$
Hence here the z-score is: $z_{72}=\frac{72-69}{2.75}=1.1$
Then, using the table: $P(z\geq 1.1)=1-0.8643=.1357$
The probability that all of them are more than 6 feet tall is: $(.1357)^3=0.002499$