abstract algebra - Degree of splitting field extensions

The problem states:

Let $f (x) = x^3+px+q$ be an irreducible cubic polynomial with rational coefficients
and let $K$ be the splitting field of $f(x)$ over $\mathbb{Q}$. Prove that $[K : \mathbb{Q}] = 3$ if and only if $-4p^3 - 27q^2$ is a square in $\mathbb{Q}$.

Here is how far I've come on the problem. Since $f$ is cubic and the degree of the extension is $3$, then non of the roots are in $\mathbb{Q}$ hence they are in $K$. Now, I am having trouble connecting this piece of information with the given value being a square in $\mathbb{Q}$.

Thanks in advance.