Every plane parallel to the xz-plane has equation of the form
y = c, where c is the y-coordinate of every point in
the plane:
Since P(5,7,-6) is a point on the plane and the y-coordinate
is 7, the equation of the plane is y = 7
In the form ax + by + cz = d, that's
0x + 1y + 0z = 7
But that is smply y = 7.
Edwin