\(Radius = \frac{R}{2}\left( {from\,figure} \right)\)
\(\frac{M}{{\frac{4}{3}\pi {R^3}}} = \frac{m}{{\frac{4}{3}\pi {{\left( {\frac{R}{2}} \right)}^3}}}\)
\( \Rightarrow m = \frac{M}{8}\)
Mass of the left over part of the sphere
\(M' = M - \frac{M}{8} = \frac{7}{8}M\)
Therefore gravitational filed due to the left over part of the sphere
\( = \frac{{GM'}}{{{x^2}}} = \frac{7}{8}\frac{{GM}}{{{x^2}}}\)