Answer
5.3 km.
Work Step by Step
We are told that the full height of the continent is H = 35 km. Let y represent the height of the continent above the surrounding mantle rock. The volume of mantle rock that is displaced is A(H-y).
The buoyant force equals the weight of mantle rock that is displaced, and this equals the weight of the continent.
$$\rho_{mantle}A(H-y)g=\rho_{continent}AHg$$
$$y=H(1-\frac{ \rho_{continent}}{\rho_{mantle}})=35km(1-\frac{2800\;kg/m^3}{3300\;kg/m^3})=5.3\;km$$
