a)

The total resistance of resistors connected in series is equal to the sum of the individual resistances:

In this problem, we have two resistors connected in series:


So the total resistance of the circuit is

b) 6.0 V
The relationship between voltage, current and resistance of a circuit is given by Ohm's law

where
V is the voltage
R is the total resistance
I is the current
In this circuit, we have
is the total resistance
I = 0.1 A is the current
So, the voltage of the battery is
