6 = ab^1 ⇒ 6 = ab ⇒ 6/a = b (1)
d = ab^3 (2)
Sub (1) into (2) for b and we have
d = a(6/a)^3
d = 216/a^2
a^2 = 216/d
a = √[216/d] = 6√[6/d]
And b = 6 / ( 6 √[6/d] ) = 1 / √[6/d] = √[d / 6] = √d / √6 = √ [6d ] / 6
So....the function is
y = 6√[6/d ] * [√ [ 6d ] / 6 ]^x