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.