Dear Vishwesh

general term in the series = ¹⁰C_r(x²)^10-r(a/x³)^r

                                                        = ¹⁰C_rx^20-5r(a)^r

for x¹⁵   20-5r =15

              r=1

  so coefficient of this term = ¹⁰C₁a

for x⁵    20-5r =5

             r=3

so coefficient of this term = ¹⁰C₃a³

now equate

   ¹⁰C₁a  = ¹⁰C₃a³

    a = 1/2√3

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