[tex]a)\frac{7x+3y}{5x+6y}=\frac{4}{5}\\
5(7x+3y)=4(5x+6y)\\
35x+15y=20x+24y\\
35x-20x=24y-15y\\
15x=9y|:3\\
5x=3y\\
b)5x=3y \Rightarrow x=\frac{3y}{5} \Rightarrow \frac{x}{y}=\frac{3}{5} \Rightarrow \\
\Rightarrow \frac{x}{3}=\frac{y}{5}=k \Rightarrow x=3k,y=5k\\
\frac{5x+8y}{7x+9y}=\frac{15k+40k}{21k+45k}=\frac{45k}{66k}=\frac{15}{22}[/tex]
