$\frac{x}{7a^2{b}^3}\ast <!--\; \ast \; -->\frac{y}{4c}=\frac{6a^3{c}^2}{b}$
$\frac{24c^3}{7a^2{b}^3}\ast <!--\; \ast \; -->\frac{7a^5{b}^2}{4c}=\frac{6a^3{c}^2}{b}$
Ответ: $x=24c^3,y=7a^5{b}^2$.

$\frac{36m^2{n}^4}{x}:\frac{y}{35p^6}=\frac{21n}{5m{p}^3}$
$\frac{36m^2{n}^4}{x}\ast <!--\; \ast \; -->\frac{35p^6}{y}=\frac{21n}{5m{p}^3}$
$\frac{36m^2{n}^4}{25p^9}\ast <!--\; \ast \; -->\frac{35p^6}{12m^3{n}^3}=\frac{21n}{5m{p}^3}$
Ответ: $x=25p^9,y=12m^3{n}^3$.