Kirchhoff's Voltage Law (KVL) states that the algebraic sum of voltages (or "voltage drops") must equal zero around a loop of a circuit. To use it, we pick a direction (usually clockwise) and trace around a loop. Every time you enter an element through it's positive terminal, add it, and every time you enter an element through its negative terminal, subtract it. Set that expression all equal to zero, and you will be able to solve for the unknown voltage drop. If you know all voltage drops in a loop, then you can use KVL to test that you have the correct values.