To determine the oxidation number of C in CO32-, we can use the following steps:

1. Assign an oxidation number of -2 to each oxygen atom in CO32-.

2. Let the oxidation number of C be x.

3. The sum of the oxidation numbers of all atoms in a neutral molecule or ion must be equal to the overall charge of the molecule or ion. In this case, the overall charge of CO32- is -2.

4. Therefore, we can write the following equation:

```

(3)(-2) + x = -2

```

5. Solving for x, we get:

```

x = +4

```

Therefore, the oxidation number of C in CO32- is +4.