Does anybody know how to solve this?

Let $f(x) = x^2 + 6x + c$ for all real numbers $x$, where $c$ is some real number. For what values of $c$ does $f(x) = 0$ have exactly $2$ distinct real roots?

This will have two distinct roots when the discriminant > 0

So

6^2 - 4 (1)(c) > 0

36 - 4c > 0

36 > 4c

36/4 > c

9 > c

It will have two distinct roots when c < 9