What is the reduction half-reaction for this redox equation? Ca + Al (NO3) 3 → Al + Ca (NO3) 2

The reduction half-reaction in this redox equation would be Al (NO3) 3 to Al. You can calculate the oxidation number of the Al in Al (NO3) 3 by noticing that it has 3 NO3 groups (each NO3 is a 1 - charge), and that the molecule has a net charge of zero. This means that the Al in Al (NO3) 3 is 3 + in oxidation state (to balance out the 3-), and it’s being reduced to 0 through the equation.

The reduction half-reaction for this redox equation is

Al³⁺ + 3e⁻ → Al