Answer:
[tex]3x + ay = 50\\ay = 50-3x\\ay = -3x + 50\\y =\frac{-3x}{a} + \frac{50}{a} \\\\slope = \frac{-3}{a} \\\\9x -21y = 15\\-21y = 15 -9x\\21y = 9x - 15\\\\y=\frac{9x}{21} - \frac{15}{21} \\\\Slope = \frac{9}{21} \\\\Since \ equations \ are \ parallel \ = m_{1} m_{2} = 1\\\\\frac{-3}{a} \cdot\frac{9}{21} = 1 \\\\\frac{-27}{21a} = 1\\\\\\\frac{-9}{7} = a[/tex]
