7.3 General Term and Middle Terms

General term $T_{r+1} = {^9C_r} (x)^{9-r} (2y)^r$. $T_{r+1} = {^9C_r} 2^r x^{9-r} y^r$ We need power of $y$ to be 3 (or $x$ to be 6). So, $r=3$. Coefficient = $^9C_3 \cdot 2^3 = \frac{9 \cdot 8 \cdot 7}{3 \cdot 2 \cdot 1} \cdot 8 = 84 \cdot 8 = 672$.