C(n+1)-PCn
=2^(n+1)-3^(n+1)-P(2^n-3^n)
=2*2^n-P*2^n-3*3^n+P*3^n
=(2-P)*2^n+(P-3)*3^n
要它是等比数列,则要:2-P=0或P-3=0
P=2或3