If $\overrightarrow a + \overrightarrow b + \overrightarrow c = 0$ and $| \overrightarrow a | = 3, | \overrightarrow b | = 5$ and $|\overrightarrow c | = 7$, show that the angle between $\overrightarrow a\: and \: \overrightarrow b$ is equal to $60^{\circ}$

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