\(F \propto V^a \rho^b g^c\)
\(F=\left[L^3\right]^a\left[ ML ^{-3}\right]^b\left[ LT ^{-2}\right]^c\)
\(\left[ ML ^{-2}\right]=F=\left[ M ^{ b } L ^{3 a -3 b +c} T ^{-2 c}\right]\)
On comparing.
\(b=1,\)
\(-2 c=-2\)
\(\Rightarrow c=1\)
\(3 a-3 b+c=1\)
\(\Rightarrow 3 a-3+1=1\)
\(\Rightarrow 3 a-2=1\)
\(\Rightarrow 3 a=3 \Rightarrow a=1\)
So, on putting all these values,
\(F=V \rho g\)