Emperor Yūryaku (雄略天皇) was the 21st imperial ruler of Japan. No firm dates can be assigned to this emperor or to his reign, but he is believed to have ruled the country during the late 5th century CE.
... square" is to add a constant (i.e., in this case, a quantity that does not depend on x) to the expression to the left of "=", that will make it a perfect square trinomial ...