Consider the the polynomial $$p(x)=x^3 - (a + 1)x^2 + (a - 6)x + 6a$$ where $x$ is the variable and $a$ is a parameter. (That means we think of it as a particular real number, but we don't specify which.) It is easy to check that $p(a) = 0.$
$p$ has two more real roots. They are
and (in increasing sequence).
Hint: Divide by $(x-a)$ and solve the resulting quadratic equation.