$$ \sqrt{7\sqrt{7\sqrt{7\sqrt{7....}}}} = x $$
$$ x = \sqrt{7}x $$
$$ x^2 = 7x $$
$$ x^2 - 7x = 0 $$
x=0,7
neglect 0 x = 7
now $$ 7 = (343)^{y-1} $$
$$ (343)^{\frac{1}{3}} = (343)^{y-1} $$
$$ \frac{1}{3} = y - 1 $$
$$ y = \frac{4}{3} $$
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