If the line segment bisector divides the original line segment into two equal pieces that are each 2.9 cm long, then we can find the total length of the original line segment by adding the lengths of the two pieces together.
Since each piece is 2.9 cm long, the original line segment is:
\[ 2.9 , \text{cm} + 2.9 , \text{cm} = 5.8 , \text{cm} \]
Therefore, the length of Andy's original line segment is 5.8 cm.