Ich mache es mal was ausführlicher:
$$y =\frac {mx^{m+n-1} - nx^{m+n-1}}{x^{2n}}$$
$$y =\frac {m * x^{m+n-1} - n * x^{m+n-1}}{x^{2n}}$$
$$y =\frac {m * x^{m}* x^{n}* x^{-1} - n * x^{m}* x^{n}* x^{-1}}{x^{n}* x^{n}}$$
$$y =\frac {x^{n}*(m * x^{m}* x^{-1} - n * x^{m}* x^{-1})}{x^{n}* x^{n}}$$
$$y =\frac {m * x^{m}* x^{-1} - n * x^{m}* x^{-1}}{x^{n}}$$
$$y =\frac {m * x^{m-1}- n * x^{m-1}}{x^{n}}$$