Sorry my bad,
I meant "dP/dh = 0, which kills the P(x) term", when I wrote "multiply by 0".
My second question is that I am confused about the prime (') in the question.
For example y' can either mean dy/dx or dy/dt, depending on the context.
Hence, my question is that in Q44, does the P''(x) in the final 5 options refer to d^2/dx^2 P(x) or d^2/dh^2 P(x).
I would think that P''(x) would be "with respect to x", but would that contradict the fact that in my working, P was being differentiated with respect to h.
Thanks for clearing my doubts..