(a,\ 2a+2,\ 3a+3,\ldots) in G.P.

[ (2a+2)^2=a(3a+3) ] [ 4(a+1)^2=3a(a+1) ]

If (a+1\neq0): (4(a+1)=3a\implies4a+4=3a\implies a=-4)

Terms: (-4,\ -6,\ -9,\ldots) with (r=\dfrac{3}{2})

Fifth term: (-4\cdot\left(\dfrac{3}{2}\right)^4=-4\cdot\dfrac{81}{16}=-\dfrac{81}{4})

Answer: (-\dfrac{81}{4})