6. In the spatial coordinate system, a curve's projections on the planes $x O z$ and $y O z$ satisfy the equations
$$
5 x + \cos z = 0, \quad z = \arctan \sqrt{y - 3} \text{. }
$$

Find a function $y = f(x)$ such that its graph consists of and only of the points that are the projections of the same curve under these conditions onto the plane $x O y$.
6. For the point $(x, y, z)$ to lie on the given curve, it is necessary and sufficient that
$$
\begin{array}{l}
\left\{\begin{array} { l } 
{ 5 x + \cos z = 0 , } \\
{ z = \arctan \sqrt { y - 3 } }
\end{array} \left\{\begin{array}{l}
\cos z=-5 x, \\
\sqrt{y-3}=\tan z, \\
-\frac{\pi}{2}<z<\frac{\pi}{2}
\end{array}\right.\right. \\
\Leftrightarrow\left\{\begin{array} { l } 
{ \cos z = - 5 x , } \\
{ y - 3 = \frac { 1 } { \cos ^ { 2 } z } - 1 , } \\
{ 0 \leqslant z < \frac { \pi } { 2 } }
\end{array} \Leftrightarrow \left\{\begin{array}{l}
5 x=-\cos z, \\
y=\frac{1}{25 x^{2}}+2, \\
0 \leqslant z<\frac{\pi}{2} .
\end{array}\right.\right.
\end{array}
$$

To describe the projection of all such points onto the $x O y$ plane, note that as the variable $z$ takes all values in the range $0 \leqslant z<\frac{\pi}{2}$, the variable $5 x=-\cos z$ takes all values in the range $-1 \leqslant 5 x<0$. At this time, $y=\frac{1}{25 x^{2}}+2$.