• made easy
• Quantity: A Real and Imaginary History of Algebra
• https://www.amazon.in/Quantity-Real-Imaginary-History-Algebra/dp/0452288533/ref=sr_1_1?crid=FPHS5S9ETR2K&keywords=unknown+quantity&qid=1653457739&sprefix=unknown+quantity%2Caps%2C291&sr=8-1&asin=B00X8EMOPO&revisionId=&format=2&depth=1
• brilliant.org