### Extension field adjoining two roots

I'm trying to construct given an irreducible polynomial $f \in \mathbb{Q}[X]$ an extension field that adjoins two of its roots $\alpha_1,\alpha_2$. I'm trying to follow the approach suggested in this question
. However, with the following code:

```
P.<x> = QQ[]
f = x^3+2*x+5 # f = P([5,2,0,0,1]) if you want
f_roots = f.roots(QQbar, multiplicities=False)
print f_roots
alpha = f_roots[0]
beta = f_roots[1]
K = QQ[alpha,beta]
K['x'](f).is_irreducible()
```

But this gives the error:

```
ValueError: defining polynomial (x^3 + 2*x + 5) must be irreducible
```

Although, the polynomial is clearly irreducible over $\mathbb{Q}$. Doing it as:

```
P.<x> = QQ[]
f = x^3+2*x+5
f_roots = f.roots(QQbar, multiplicities=False)
alpha = f_roots[0]
K.<a> = QQ[alpha]
beta = f_roots[1]
K1.<b> = K[beta]
```

Gives error:

```
ValueError: base field and extension cannot have the same name 'a'
```

What is going wrong? Is this the right way to construct the extension field with two roots?

**Edit**

Let me emphasize that I'm looking for a method that works for an arbitrary degree not just for degree 3. My actual problem goes on a degree four polynomial. So take as an example:

~~$f ~~f = x^4+2*x+5 ~~$~~

instead of the previous one.