Stauracius was a Byzantine Emperor and son of Nicephorus I. Paralyzed by a sword wound to his neck while fighting against the Bulgars[?] at the Vurbitsa[?] pass, he reigned for one month, before retiring to a monastery and dying five months later.
... 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 of the form x2 + ...