Question

Why did they choose x for unknown?

Answer 1

x is an arbitrary variable in an elementary equation such as:
y = 2x

u or any other variable could be used in place of x, but consistency is key!
y = 2u

Answer 2

guess this might help...
http://www.lifeslittlemysteries.com/1587-x-represents-unknown-quantity-variable.html

Answer 3

It is because when Algebra was discovered, it was discovered by Muhammad ibn Mūsā al-Khwārizmī and Arab Mathematician in the 7 th Century. When X is translated into Arabic it means "thing". Thus when algebra spread to the Western World, the Arabic word of "thing" was translated into what we know today as "X".