if (3-a) 6(2-5a) are conservative terms of a GP with common ratio r>0.

Question

find the value of a

Answer 1

I will assume you meant to say:
3-a , 6, and 2-5a are consecutive terms of a GP,

(3-a)(2-5a) = 36
6 -17a + 5a^2 - 36 = 0
5a^2 - 17a - 30 = 0
a = (17 ± √889)/10