Final answer:
The equation for the cross section at y = -7 is x² + z² = 7, which represents a circle with center at the origin (0, 0) and radius √7.
Step-by-step explanation:
To find the equation for the cross section at y = -7, we need to substitute y with -7 in the given equation x² + z² = 7.
So, the equation becomes x² + z² = 7.
This equation represents a circle with center at the origin (0, 0) and radius √7.