If the coefficient of x ^ 3 in the binomial expansion of (x ^ 2 + 1 / ax) ^ 6 is 5 / 2, then a =? explain

Let K be x ^ 3
Then C6 (k-1) * (x ^ 2) ^ (6-k + 1) * (1 / ax) ^ (k-1)
Index = 2 (6-k + 1) - (k-1) = 3
15-3k=3
k=4
So the coefficient = C63 * (1 / a) ^ 3 = 5 / 2
(1/a)^3=1/8
a=2

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