Gravitational field strength is given by the formula
[tex]g = \frac{GM}{R^2}[/tex]
now another planet Zeta is of same mass but double the gravitational strength
[tex]2g = \frac{GM}{r^2}[/tex]
now plug in the value of g from above
[tex]2\frac{GM}{R^2} = \frac{GM}{r^2}[/tex]
by solving above equation for radius "r"
[tex]r = \frac{R}{\sqrt2}[/tex]
so its radius is 0.707R