$(\frac{1}{4}ab^2c^3-\frac{7}{9}ab^2c^3+ab^2c^3):(-\frac{17}{18}abc^3)$
Facciamo un raccoglimento totale, raccogliamo $ab^2c^3$:
$^b\cancel{ab^2c^3}(\frac{1}{4}-\frac{7}{9}+1) \cdot \frac{18}{17\cancel{abc^3}} \cdot (-1)$
$-b(\frac{\cancel{17}}{\cancel{36}_2})\cdot \frac{\cancel{18}}{\cancel{17}}$
$-\frac{b}{2}$
Ecco fatto!
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$ \left(\dfrac{1}{4}ab^2c^3-\dfrac{7}{9}ab^2c^3+ab^2c^3\right) : \left(-\dfrac{17}{18}abc^3\right)=$
$ =\left(\dfrac{9-28+36}{36}ab^2c^3\right) : \left(-\dfrac{17}{18}abc^3\right)=$
$ = \dfrac{17}{36}ab^2c^3 : \left(-\dfrac{17}{18}abc^3\right)=$
$ = \dfrac{17}{36}·\left(-\dfrac{18}{17}\right)\dfrac{ab^2c^3}{abc^3}=$
$ = \dfrac{\cancel{17}^1}{\cancel{36}_2}·\left(-\dfrac{\cancel{18}^1}{\cancel{17}_1}\right)\dfrac{\cancel{a}b^{\cancel2}\cancel{c^3}}{\cancel{a}\cancel{b}\cancel{c^3}}=$
$ = -\dfrac{1}{2}b\quad \left(= -\dfrac{b}{2}\right)$