How do you solve y^2 - 4y - 21 = 0 using the quadratic formula?

${x}_{1} = 7$
${x}_{2} = - 3$

Explanation:

So basically the quadratic equation is $x = \frac{- b \pm \sqrt{{b}^{2} - 4 a c}}{2 a}$
Here $x = y , a = 1 , b = - 4 , c = - 21$
Plug that in and we get:
$x = \frac{- \left(- 4\right) \pm \sqrt{{4}^{2} - 4 \cdot 1 \cdot \left(- 21\right)}}{2 \cdot 1}$

$x = \frac{4 \pm \sqrt{16 + 84}}{2}$

$x = \frac{4 \pm \sqrt{100}}{2}$

$x = \frac{4 \pm 10}{2}$

$x = 2 \pm 5$

There are two possible values for $x$.
${x}_{1} = 7$
${x}_{2} = - 3$