What does the negative sign in the expression $E_{Zn^{2+}|Zn}^o = -0.76 \ V$ mean?

If $Cu^{2+} + 2e^{-} \rightarrow Cu, E^{0} = 0.337 \ V$ and $Cu^{2+} + e^{-} \rightarrow Cu^{+}, E^{0} = 0.153 \ V$,then for the reaction $Cu^{+} + e^{-} \rightarrow Cu$,$E^{0}_{cell} =$ .............. $V$.

If the cell potential of the cell at $298 \ K$ is $2.36 \ V$,write the cell reaction and calculate the standard electrode potential of the $Mg^{2+} \mid Mg$ half-cell.
$Mg_{(s)} \mid Mg^{2+}_{(1 \ M)} \parallel H^{+}_{(1 \ M)} \mid H_{2(g)} (1 \ bar) \mid Pt_{(s)}$

How would you determine the standard electrode potential of the system $Mg^{2+} | Mg$?

The cell reaction of a cell is $Mg_{(s)} + Cu^{2+}_{(aq)} \to Cu_{(s)} + Mg^{2+}_{(aq)}$. If the standard reduction potentials of $Mg$ and $Cu$ are $-2.37 \ V$ and $+0.34 \ V$ respectively,the $EMF$ of the cell is ................. $V$.

Calculate the standard cell potential for a cell having the following reaction: $2 Al_{(s)} + 3 Ni^{2+} \rightarrow 2 Al^{3+} + 3 Ni_{(s)}$ given that $E_{Ni^{2+}/Ni}^{\circ} = -0.25 \ V$ and $E_{Al^{3+}/Al}^{\circ} = -1.66 \ V$. (in $V$)