813 -7?

806

Explanation: 813 − 7 requires borrowing in the units place: 3 − 7 isn’t possible, so borrow 1 from the tens (making 1 become 0 and the 3 become 13). Then 13 − 7 = 6, and the hundreds digit stays 8, giving 806.

If you meant a different expression (e.g., 813 − 7x or something), tell me and I’ll adjust. Want more practice problems?