Find the value of 𝑥 - Algebra Can be Fun 

if the sides are equal, and the angle is 90 degrees, you can rotate one triangle to get a rectangle

Where does the 100x come from? The other numbers - 900, 2,500 & 400 - are fairly obvious, if someone is watching the video they probably don't understand were the 100x came from; if they do they are probably not watching the video.