Answer
50.0 volts.
Work Step by Step
Use the equation right before equation 31.20, which relates the rms voltages.
$$V=\sqrt{V_R^2+(V_L-V_C)^2}$$
$$V=\sqrt{(30.0V)^2+(50.0V-90.0V)^2}$$
$$V=50 volts$$
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