u c, x1=a, x2=ar, x3=ar^2, x4=ar^3.
so from first eqn, a(1+r)=3 --------------1,
a^2r=p ------------------2
from second, ar^2(1+r) = 12---------------3,
a^2r^5=q --------------4
divide 3 by 1...
so u have r^2=4 =>r = +-2
now wen u got r, u will get a..
then u will get p and q...