Three Letters, Infinite Power

Ohm's Law is V = IR: voltage equals current times resistance. Know any two quantities and you can calculate the third, which is why the equation runs electrical engineering.

Rearrange it three ways and you have a complete toolkit: V = IR to find voltage, I = V/R to find current, R = V/I to find resistance. A 12V battery pushing current through a 6-ohm resistor gives 12 / 6 = 2 amperes. Once you know two values, the third is never a mystery.