$$(\frac{2}{a} - \frac{3}{x}) : \frac{ax^2}{3a-2x} = \frac{2x-3a}{ax} \cdot \frac{3a-2x}{ax^2} = - \frac{(3a-2x)^2}{a^2x^3}$$
При $$a = 4,56$$ и $$x = -12,34$$:
$$-\frac{(3 \cdot 4,56 - 2 \cdot (-12,34))^2}{(4,56)^2 \cdot (-12,34)^3} = -\frac{(13,68 + 24,68)^2}{(20,7936) \cdot (-1875,243304)} = -\frac{(38,36)^2}{-38991,66817} = -\frac{1471,4896}{-38991,66817} = 0,0377$$
Ответ: 0,0377