Answer:
[tex]r = \sqrt[3]{\frac{4}{3} }[/tex]
Step-by-step explanation:
[tex]-r + (3r^2 + 1)r = 4\\-r + 3r^3 + r = 4\\3r^3 = 4\\r^3 = \frac{4}{3} \\\\r = \sqrt[3]{\frac{4}{3} }[/tex]
