Answer
$L\approx1.53cm$
Work Step by Step
We can use the given values from the lab measurements and the length contraction equation to find the length as measure from the electron's reference frame.
$L=L_0 \sqrt{1-\beta^2}$
$L=3.00m \sqrt{1-(0.999987c)^2}$
$L \approx 0.0153m$ or $1.53 cm$
$L\approx1.53cm$
