It's just basic algebra. You’ve known it for years already, just by other names.

Remember your basic algebra classes where you learned what was done to one side of the equation was also done to the other.

Well, the bookkeeping world has fancy names for the left side and right side of the accounting equation. The left side means debit. The right side means credit. That’s it!

You also learned in algebra about formulas and variables. You understood what the variables meant and which formula to use, you plugged in numbers and solved the equation.

Well, know the accounting equation!

Assets = Liabilities + Equity

Equity = Owner contributions + Owner Withdrawals + Net Income

Net Income = Revenue + Gains - Expenses - Losses

Your example,

300 = 300 + 0

-300 = -210 - 90