\(M \propto N \) ; \(M \propto \frac{1}{\sqrt{P}}\).
\(\therefore M \propto \frac{N}{\sqrt{P}}\)
\(M = \frac{k N}{\sqrt{P}}\)
when M = 3, N = 5 and P = 25;
\(3 = \frac{5k}{\sqrt{25}}\)
\(k = 3\)
\(M = \frac{3N}{\sqrt{P}}\)
when M = 2 and N = 6,
\(2 = \frac{3(6)}{\sqrt{P}} \implies \sqrt{P} = \frac{18}{2}\)
\(\sqrt{P} = 9 \implies P = 9^2\)
P = 81