The Total solution for NCERT class 6-12
Let a = k + 9; b =k−6; and c = 4;
We know that a, b andc are in GP, then
b2 =ac {using property of geometric mean}
(k − 6)2 = 4(k+ 9)
k2 –12k + 36 = 4k + 36
k2 –16k = 0
k = 0 or k = 16