$\"\"$

The equation is $\"\"$ .

Compare the above equation with standard form of quadratic equation $\"\"$ .

a = 4, b = $\"\"$ 4, and c = 21.

The quadratic formula : $\"\"$ .

$\"\"$ (Substitute : a = 4, b = $\"\"$ 4, and c = 21)

$\"\"$

$\"\"$ (The imaginary unit $\"\"$ )

$\"\"$

The solution of $\"\"$ .