On proposition I.3.2 of 'Quantum groups' by Kassel. I am reading the book Quantum groups by Kassel. In proposition I.3.2 at the very beginning the reader is asked to show that under the identifications made, the maps $\Delta,\varepsilon$ and $S$ correspond to the maps $+,0$ and $-$.
However, $+$ is a map from $A^2$ to $A$, and I'm not sure how we can use the identifications to see that $\Delta:k[x]\rightarrow k[x',x'']$ corresponds to $+$.
It seems to me, all you can get from the identifications is that the map $\Delta$ corresponds to the element $x'+x''\in k[x',x'']$. Similarly, $\varepsilon$ corresponds to the element $0\in k$ and $S$ corresponds to the element $-x\in k[x]$.
So what exactly is meant in this proposition? I'm fairly sure this is a stupid question, but it's one that should be well-understood before proceeding any further in this theory.
Thank you in advance.

$\text{Hom}(k[x], -)$ is the forgetful functor from $k$-algebras to sets; that is, a homomorphism $k[x] \to k[x', x'']$ of $k$-algebras is the same thing as an element of $k[x', x'']$, namely the image of $x$. Alternatively, the point is that a morphism $A \times A \to A$ of affine varieties over $k$ (here $A$ is the affine line) dualizes to a morphism $k[A] \to k[A] \otimes k[A]$ in the opposite category, and $k[A] \cong k[x]$ while $k[A] \otimes k[A] \cong k[x', x'']$. The other cases are similar. 
This can be used to set up various equivalences; for example, you get a contravariant equivalence of categories between commutative Hopf algebras over $k$ and affine group schemes over $k$. 
