Subtract exponent in denominator

\dfrac{a^m}{a^n} = a^{m-n}

\begin{pmatrix} n \\ 0 \end{pmatrix}x^ny^0 + \begin{pmatrix} n \\ 1 \end{pmatrix}x^{n-1}y^1+ \begin{pmatrix} n \\ 2 \end{pmatrix}x^{n-2}y^2+\dots+ \begin{pmatrix} n \\ n-1 \end{pmatrix}x^1y^{n-1}+ \begin{pmatrix} n \\ n \end{pmatrix}x^0y^n

(x+y)^8 = x^8 +8x^7y+28x^6y^2+56x^5y^3+70x^4y^4+56x^3y^5+28x^2y^6+8xy^7+y^8