Final answer:
The necessary general entry to record the purchase of the machine is a debit to Machine for $30,000, a credit to Cash for $18,000, and a credit to Note Payable for $12,000.
Step-by-step explanation:
The general entry necessary to record the purchase of the machine is as follows:
Debit Machine (Asset) $30,000
Credit Cash (Asset) $18,000
Credit Note Payable (Liability) $12,000
This entry reflects the purchase of the machine for $30,000, with $18,000 paid in cash and $12,000 to be paid in the future through a note payable.