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Expand  (3 + 5x)^7

78125 x^7 + 328125 x^6 + 590625 x^5 + 590625 x^4 + 354375 x^3 + 127575 x^2 + 25515 x + 2187
(8 terms)

$\quad (3 + 5x)^7\\ = \displaystyle \sum^7_{k = 0}\binom7k \cdot 3^{7 - k} \cdot (5x)^k\\ = \displaystyle \sum^7_{k = 0} \binom7k \cdot 3^7 \cdot \left(\dfrac{5}{3}\right)^k\cdot x^k\\ \text{When }k = 2,\\ \quad\text{Term}\\ = \displaystyle\binom72 \cdot 3^7 \cdot \left(\dfrac{5}{3}\right)^2 \cdot x^2\\ = 21 \cdot 243 \cdot 25 x^2\\ \quad\text{Coefficient of }x^2\\ = 21\cdot 243\cdot 25\\ =127575$

Max: How do you get :(5/3)^2 =25? . It should be just 5^2 =25.