Answer :
[tex]\begin{gathered} \Rightarrow x(56+9b)=12b-24 \\ \Rightarrow78\times(56+9b)=12b-24 \\ \Rightarrow4368+702b=12b-24 \\ \Rightarrow702b-12b=-4368-24 \\ \Rightarrow690b=-4392 \\ \Rightarrow b=-6.36 \end{gathered}[/tex]