What is the power dissipated by a resistor
From what you said I guess that the schematic is this:







simulate this circuit &ndash; Schematic created using CircuitLab

The resistors are in series, so you need to calculate the power using the correct voltage. The voltage \$V\$ is the voltage across both resistors and in the schematic is called \$Vac\$.

You need to remember that \$V = Vac = Vab + Vbc\$.

In order to calculate the power dissipated by R1 you need to use \$Vab\$, that is the voltage across R1.