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We don't always necessarily want a positive cell potential: if we want a spontaneous reaction, we would need a positive cell potential, but that's not what we're looking for here. The first reaction is oxidized because we need to reverse that reaction to calculate Ka. The reaction that is associated with Ka breaks down an acid into its constituent components, so the products are the individual ions while the reactant is the acid itself. Notice this is the reverse of what we are given, so, we reverse the first reaction. K is Ka in this context. Ka is the acid dissociation constant - the equilibrium constant for the dissociation (breaking down) of an acid.
As Maggie stated above, a positive cell potential is not always required (it's only for when you want a spontaneous reaction). The reason why we know we want HF on the reactant side and the H+ and F- ions on the product side is because we are asked for the Ka value, which implies that the acid (in this case, HF) is dissociating into its ions. The way to end up with HF on the reactant side in order for it to dissociate, we need to flip the first equation.
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