Let $G$ be a solvable group and $X$ is a $G$-set which is transitive and faithful, with $|X|=p$ is prime. The aim is to prove
There is an injective homomorphism $i:G\to Aff(\mathbb{F}_p)$ and bijective $j:X\to\mathbb{F}_p$ such that $j(g\cdot x)=i(g)\cdot j(x)$.
(Intuitively, we want to give the set $X$ a group structure -- which then must be isomorphic to $\mathbb{F}_p$ -- such that the $G$-action is just an affine map, i.e., of the form $x\mapsto ax+b$.)
Let $G=G_n\triangleright\cdots\triangleright G_0=\{1\}$ be the composition series of $G$ (so $G_{i+1}/G_i$ is cyclic of prime order). I've proved that each $G_k$ action on $X$ is transitive and faithful. So we can proceed by induction on $n$, assuming the analogous result is true for $G_0,\ldots,G_{n-1}$.
For $n=1$, in fact the injection $i_1:G_1\to Aff(\mathbb{F}_p)$ can be chosen so that $i_1(g)$ is an affine map of the form $x\mapsto x+b$. Then I also proved $G_1\triangleleft G_n$. Let $x_0=j^{-1}(0)$. Consider the subgroup $H=Stab(x_0)$ of $G_n$. This is where I get stucked:
Prove that there is an injective homomorphism $\tilde{i}:H\to Aff(\mathbb{F}_p)$ such that $j(g\cdot x)=\tilde{i}(g)\cdot j(x)$ for all $g\in H,x\in X$, and, furthermore, $\tilde{i}(g)$ is an affine map of the form $x\mapsto ax$ for all $g\in H$.
If this is proved, I think I can finish the rest of the proof.