Hallo
$$ \frac{a\cdot 4x^{a-1}\cdot y^b }{b\cdot y^{b-1}\cdot 4x^a} = \frac{a\cdot 4x^{a}\cdot\frac{1}{x}\cdot y^b }{b\cdot y^{b}\cdot \frac{1}{y}\cdot 4x^a} = \frac{a\cdot 4x^{a}\cdot y^b }{b\cdot y^{b}\cdot 4x^a} \cdot\frac{1}{x} \cdot\frac{1}{\frac{1}{y}} = \frac{a\cdot 4x^{a}\cdot y^b }{b\cdot y^{b}\cdot 4x^a} \cdot\frac{y}{x} = \frac{ay}{bx}$$
Grüße