Given that the first three terms of the expansion are 729, 7290 and 30375, respectively.

Now T₁ = ⁿC₀ * a^n-0 * b⁰ = 729

=> aⁿ = 729  ................1

T₂ = ⁿC₁ * a^n-1 * b¹ = 7290

=> n* a^n-1 * b = 7290 .......2

T₃ = ⁿC₂ * a^n-2 * b² = 30375

=> {n(n-1)/2}* a^n-2 * b² = 30375 .......3

Now equation2/equation1

      n* a^n-1 * b/aⁿ  = 7290/729

=> n*b/n = 10 .......4

Now equation3/equation2

{n(n-1)/2}* a^n-2 * b² /n* a^n-1 * b = 30375/7290

=> b(n-1)/2a = 30375/7290

=> b(n-1)/a = (30375*2)/7290

=> bn/a - b/a = 60750/7290

=> 10 - b/a = 6075/729             (60750 and 7290 is divided by 10)

=> 10 - b/a = 25/3                     (6075 and 729 is divided by 243)

=> 10 - 25/3 = b/a

=> (30-25)/3 = b/a

=> 5/3 = b/a  

=> b/a = 5/3 .................5

Put this value in equation 4, we get

n * 5/3 = 10

=> 5n = 30

=> n = 30/5

=> n = 6 

Now put this value in equation 1, we get

a⁶ = 729

=> a⁶ = 3⁶

 => a = 3

Now from equation 5

b/3 = 5/3

=> b= 5

So value of a, b and n are 3, 5, 6 respectively.