For the Amateur Radio license exams (for most countries) you need to remember Ohm’s Law, which is easily remembered by the Ohm’s Law triangle:
V I R
where:
V = voltage (volts)
I = current (amps)
R = resistance (ohms)
To calculate any value knowing the other 2, cover the value you need with your finger and then use the remaining calculation, e.g.
V = I x R
I = V / R
Similarly for Power, use the PIV triangle:
P I V
where:
P = power (watts)
I = current (amps)
V = voltage (volts)