Solve by completing the square a^2 + 10a+21=0

A^2 + 10a + 21 = 0

a^2 + 10a = - 21

a^2 + 10a + 25 = - 21 + 25

(a + 5) ^2 = 4

sqrt (a+5) ^2 = sqrt 4

a + 5 = (+-) 2

a = - 5 (+-) 2

a = - 5 + 2 = - 3

a = - 5 - 2 = - 7

solution is : a = - 3 and a = - 7 <==