Let FV be the viscous force and FB be the buoyant force acting on the ball.

Then, when body moves with constant velocity,
FV+FB=Mg
Or, FV=Mg−FB
FV=ρBVg−ρLVg =(ρB−)(ρL)Vg, where, V is the volume of small ball.
=(8−1.3)×103×8×1030.3×10−3×10
=86.7×0.3×10−2
=867×3×10−4=25.125×10−4=25×10−4N
Thus, the value of x=25.