Find the first three terms of a geometric progression where all terms are positive, S4 (the sum to four terms) = 21 2/3 and the sum to infinity = 27

The formulas used for GP

Sn=a(1-(r^n))/(1-r) for n terms...(2)

and for infinite terms

Si=a/(1-r) for 0<r<1.......(1)

a=1st term

r=constant ratio

use it in above

put (1) in (2) for n= 4 to get r and thus a

further use (2) to find the terms reqd