Respuesta :

mohabbasem
First you calculate the determinant:
d=b^2-4ac
d=4^2-4*1*(-21)=16+84=100

Because d>0 there are two solutions for x:

x_1 = (-b+√d)/2a = (-4+√100)/2*1 = 6/2 = 3
x_2 = (-b-√d)/2a = (-4-√100)/2*1 = -14/2 =-7

The solutions for x is x = 3 and x = -7