RELATIVE MOTION wooden boxcar, sniper, bullets oh my

[auk411]

Homework Statement

A wooden boxcar is moving along a straight railroad track at speed v1. A sniper fires a bullet (initial speed v2) at it from a high powered rifle. The bullet passes thought both lengthwise walls of the car, its entrance and exit holes being exactly opposite each other as viewed from within the car. From what direction, relative to the track, is the bullet fired from? Assume that the bullet is not deflected upon entering the car, but that its speed is decreased by 20%. Take v1 to be 85 km/h and v2 = 650 m/s.

Homework Equations

The Attempt at a Solution

I don't understand what is going on. Can someone please explain the explanation to me. For example, why is the triangle set up as it is. Why is the hypotenuse the speed of the bullet as it is leaving the boxcar? And why do we have to subtract from 180 degrees.

 

[member 553083]

I understand that the bullet is fired from an angle that is perpendicular to the track, but I don't understand why we have to use the Pythagorean theorem and why we are subtracting from 180 degrees. Can someone please explain this to me. Thanks!