What does "compute Aut(G)" mean?

[Hill]

Homework Statement

Let G = {a+b√5 : a,b ∈ Q} ⊂ R.
Prove that G+ = G is a subgroup of R under addition, and compute Aut(G+).

Relevant Equations

Aut(G+)

I know that Aut($G^+$) is the group of automorphisms on $G^+$. But what does it mean to "compute" it?