Complex sentence: So in order to hit the mirror, the travel path of the beam is L in y direction and vT_3 in x direction, therefore the Pythagorean theorem gives the actual beam travel distance of\\ scriptstyle\\ sqrt.
Split into simpler sentences: So in order to hit the mirror, the travel path of the beam is L in y direction and vT_3 in x direction. Therefore the Pythagorean theorem gives the actual beam travel distance of\\ scriptstyle\\ sqrt.