Good evening.
This is a incomplete quadratic equation, because it does not have the term bx. Therefore, we can solve this faster with the following strategy:
[tex]\mathsf{x^2 - 16 = 0}[/tex]
Add 16 to both sides:
[tex]\mathsf{x^2 - 16 + 16 = 0 + 16}\\ \\ \mathsf{x^2 = 16}[/tex]
Take the square root:
[tex]\mathsf{\sqrt {x^2} = \pm\sqrt{16}}\\ \\ \mathsf{\boxed{\mathsf{x = \pm4} }}[/tex]
Therefore:
[tex]\boxed{\boxed{\mathsf{S=\{-4, \ +4\}}}}[/tex]
