Answer: -2/7

Line $\ell$ passes through $(0, 0)$, so its equation is $y = mx$, and for any point $(a, b)$ on it with $a \ne 0$, $\frac{b}{a} = m$. The graph of $2x + 7y = 7$ can be written as $y = -\frac{2}{7}x + 1$, and parallel lines have equal slopes. So $m = -\frac{2}{7}$ and $\frac{b}{a} = -\frac{2}{7}$.

Answering $-\frac{7}{2}$ uses the reciprocal of the slope, but $\frac{b}{a}$ is rise over run from the origin, which is the slope itself.