Answer
$$x \approx 15$$
Work Step by Step
$b = x$
$$x^2 = a^2 + c^2 - 2ac \space cos \space \theta$$ $$x = \sqrt {(24)^2 + (30)^2 - 2(24)(30) \space cos \space 30 ^o}$$ $$x \approx 15$$
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