Answer
32 $\Omega$
Work Step by Step
As discussed in example 19-3 or in Problem 12, with 8 identical resistors in series, each one has one-eighth of the total voltage across it, $V=\frac{120V}{8}=15V$.
This voltage, and the power of each bulb, will allow us to calculate the resistance of a single bulb.
$$P=\frac{V^2}{R}$$
$$R=\frac{V^2}{P}=\frac{(15\;V)^2}{7.0W}=32\Omega$$
