The formula is A*V = W. For example, if you have a current of 2 A and a voltage of 5 V, the power is 2A * 5V = 10W.
The formula is W/V = A. For example, if you have a power of 10W running at 5V, the current is 10W / 5V = 2A.
The formula is W/A = V. For example, if you have a power of 10W with a current of 2A the voltage is 10W / 2A = 5V.