[tex]\it T_{k+1}=C^k_{15}(x^{\frac{2}{3}})^{15-k}\cdot(xa^\frac{2}{3})^k\\ \\ a^{\frac{2k}{3}}=a^2 \Rightarrow\dfrac{2k}{3}=2|_{:2}\Rightarrow \dfrac{k}{3}=1 \Rightarrow k=3[/tex]
