Ohm's Law and in-series components
You need to use KVL around the loop:







simulate this circuit &ndash; Schematic created using CircuitLab

KVL says that the sum of the voltages around the loop equals 0. That means that if there is a 12V increase traveling from the negative to the positive terminal of the voltage source, then the voltage must fall by 12V traveling through the bulb and resistor.

The current in the loop can be found using Ohm's Law and from the voltage \$V\$ across the \$10\Omega\$ resistor you found in part (a).

The resistance of the bulb can then be found using Ohm's Law again: you are given the fact that 8V is across the bulb and from part (b) you know the current through it. Solve for the resistance \$R\$.