Why 'x' is unknown number?

For hundreds of years, the 'x' symbol has been used to mean unknown numbers in mathematical terms. The question is, why is x chosen? How did it relate to mathematics?
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Musa Al Khawarijmi, father of the algebra, was one of the early directors of 'House of Wisdom' founded in Baghdad during the reign of Abbasid Khilafat. During his time, the light of Islamic civilization and culture in Iberian Peninsula (present-day Spain) spread through the Western African Muslims. A few days ago, Radius Foundation's director Terry Moore presented the history of using x as an unknown number in a talk at Ted Talk. He said that the introduction of the x as an unknown number began with the form of helplessness in some letters of the Arabic translation by the Spanish scholars. One such character is 'Shin'. The Arabic word 'al-shalan' is used to mean

something unknown. Initially, the word was very popular in mathematics (because, according to Arab mathematicians, the use of Hindu-Arab numerology and '0' in Europe began to be used, and hence the effect of Arabic words on European education was greatly appreciated.)
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The Spanish scholars had no letters similar to translating the Arabic 'sh' sound. As a result, they translate this sound by 'ck', which is expressed in Classical Greece by the 'chi' symbol. Later this translation of 'chi' was replaced by the Latin letter 'x' when translating into Latin.
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As an example, we see Xmas, the original word is christmas, in which case Christian religious scholars used x instead of the Greek 'chi'.
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In the 1909-1916 version of the Webster Dictionary, it has been repeated in the same theory (Theory is said to be that, according to many, there is no written history of this connection to the Arabs with x). It has been said that the word 'Shee' is used in Arabic to mean 'things', and it is translated into Greek by the word 'xei', whose short form is x ..... Ali Khansari further mentions that the word xenos is used to mean 'strangers' in Greek, which started with x characters
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And thus, the introduction of x as a symbol of 'strangers' by translating from Arab Muslims to Spanish and later in Greek and Latin.