LGN I believe. If you look at how the ret part of the match corresponds with the next match, you can correspond the letters based on the other triangles.
{3,4,5,6} should be the answer
Square centimeters are just telling that it's an area of a rectangle, square or some other shape.
In an equation using x is just a place holder because those numbers are variables so it doesn't matter whether it is x or w.
