a summary of ted talk- why X always represent unknown?

Why is it that the letter Xrepresents the unknown? Firstly it appaear in math class, but now has been in culture. And it has become a problem to the speaker and tries to find the answer. He got the answer when firstly leaned Arabic language. As we know that, Arabic language is very complicated, every part of it has meaning and information. He also said to write a word or a phraseor a sentence in Arabicis like crafting an equation. In golden century of islam, there are a golden knowledge, books were written in Arabic and translated by many countries such as spain and America. But there is a problem when translating them, because not all Arabic letter is in Spanish language. For example: the word Shalan, the letter Sh, Sheen, It's also the very first letter of the word shalan,which means "something"just like the the English word "something" —some undefined, unknown thing.So as-Shalan means the unknown thing. And when medieval Spanish try to translate it into their language, it becomes a problem because their language doesn’t have such sound, then they simply changed it into ‘ck’ sound, which is borrowed from greeklatin. Later, when the material is translated into European language, lets say latin, they also simply replaced the word and sound ‘ck’ with X, so that’s why, now X is represent as unknown thing, from the word Shalan (means something), Sh to  Ck  to   X, it is also because they cannot say Sh in Spanish.