Given, equations of curves are
x2 = 9p(9 - y) ....(i)
and x2 = p(y + 1) .....(ii)
As, these curves cut each other at right angle, therefore their tangent at point of intersection are perpendicular to each other.
So, let us first find the point of intersection and slope of tangents to the curves.
From Eqs. (i) and (ii), we get
9p(9 - y) = p(y + 1)
9(9 - y) = y + 1
Hence, the value of P is 4