Respuesta :

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 <==