Every transaction leaves the accounting equation in balance: assets equal liabilities plus equity. Assets are resources the business controls. Liabilities are amounts it owes. Equity is the owner's residual claim after liabilities. Revenue increases equity, expenses reduce it, and owner contributions and withdrawals change equity without changing profit.
Start with what happened, then trace both sides. Buying a computer for cash swaps assets and leaves total assets unchanged. Buying it on credit adds an asset and a liability. A business can have negative equity when its liabilities exceed its assets; the equation still works. Use signed changes when you calculate a new balance.