[tex]\displaystyle{ \frac{x}{y} = \frac{3}{5} \rightarrow 5x = 3y \rightarrow x = \frac{3y}{5} }[/tex]
[tex]\displaystyle{ \frac{3x + 7y}{4x + 2y} = \frac{3 \cdot \frac{3y}{5} + 7y}{4 \cdot \frac{3y}{5} + 2y} }[/tex]
[tex]\displaystyle{ =\frac{4,5y + 7y}{2,4y + 2y} = \frac{11,5y}{4,4y} }[/tex]
[tex]\displaystyle{= \frac{11,5}{4,4} }[/tex]
