Ohm's law states that in an electrical circuit, the current passing through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them. But you don't want to know that!
What the Law tells you is that in knowing two values of a circuit, the third can be found.
Before you start though you will need to know this
Power is measured in Watts and is represented by W
Current is measured in Amperes and is represented by I
Resistance is measured in Ohma and is represented by R
Voltage is measured in EMF and is represented by E

Find the Voltage by the following formulae:
E = I x R
E = W / I
√W x R
Find the Resistance by the following formulae:
R = E / I
R = E x E / W
R = W / I x I
Find the Current by the following formulae:
I = E / R
I = W / E
I = √W / R
Find the Power by the following formulae:
P = E x I
P = I x I x R
P =E x E / R