3. $A$ is a point on the parabola $x=-\frac{2}{7} y^{2}$, $F$ is the focus, $|A F|=14 \frac{7}{8}$, the equation of the line $l$ passing through point $F$ and perpendicular to $O A$ is $\qquad$ .
$3.8 x-4 y+7=0$ or $8 x+4 y+7=0$.
Let $A\left(x_{1}, y_{1}\right)$. By definition, $14 \frac{7}{8}=\left|x_{1}\right|+\frac{7}{8}$.
Then $A(-14,7)$ or $(-14,-7)$.
Thus $k_{O A}=\frac{1}{2}$ or $-\frac{1}{2}$.