Using the quadratic formula, find the roots of x^2 - 4x - 21=0

=
−

±

2
−
4

√
2

x
=
−
b
±
b
2
−
4
a
c
2
a
x=2a−b±b2−4ac
Once in standard form, identify a, b and c from the original equation and plug them into the quadratic formula.

2
−
4

−
2
1
=
0
x
2
−
4
x
−
21
=
0
x2−4x−21=0

=
1
a
=
1
a=1

=
−
4
b
=
−
4
b=−4

=
−
2
1
c
=
−
21
c=−21

=
−
(
−
4
)
±
(
−
4
)
2
−
4
⋅
1
(
−
2
1
)
√
2
⋅
1

Hope i can you!