As dori sa ma ajute cineva cu aceasta tema(la ex 3 rezolvare)

[tex]\it 5a+3c=13\ \ \ \ \ \ (*)\\ \\ 7b+5ab+3bc=140\ \Rightarrow\ 7b+b(5a+3c)=140\ \stackrel{(*)}{\Longrightarrow}\ 7b+13b=140\ \Rightarrow\ \\ \\ \ \Rightarrow\ 20b=140|_{:20}\ \Rightarrow\ b=7[/tex]