x = 1/6 (7+(721-84 sqrt(57))^(1/3)+(721+84 sqrt(57))^(1/3))
\(x = \frac{1}{6}\left[7+(721-84 \sqrt{57})^{1/3}+(721+84 \sqrt{57})^{1/3}\right]\\ \)
You can get an approximation on your calculator but this cannot be simplified.
1/6 (7+(721-84 sqrt(57))^(1/3)+(721+84 sqrt(57))^(1/3)) = 3.7490191400643459