$C=\dfrac{7}{9}\left(x^2y^{\tfrac{3}{2}}\right)^2.\dfrac{18}{21}\left(x^{\tfrac{6}{2}}\dfrac{1}{y}\right)^2+2+2x^2y^5\\ =\dfrac{7}{9}.\dfrac{2.9}{3.7}x^{2.2} y^{\tfrac{3}{2}.2}.x^{\tfrac{6}{2}.2}\dfrac{1}{y^2}+2+2x^2y^5\\ =\dfrac{2}{3}x^4 y^3.x^6.\dfrac{1}{y^2}+2+2x^2y^5\\ =\dfrac{2}{3}x^{10}y+2+2x^2y^5(*)\\ x=1,y=2 \Rightarrow (*)=\dfrac{2}{3}.1^{10}.2+2+2.1^2.2^5=\dfrac{202}{3}$
