First, we need to show that $F(c) \subseteq F(ac+b)$. To do this, we need to show that any element in $F(c)$ can be expressed as a linear combination of elements in $F(ac+b)$.
Let $x \in F(c)$. Then, $x = p(c)$ for some polynomial $p(t) \in F[t]$. We can write
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