Đáp án:
$\begin{array}{l}
b = \dfrac{{0,9 + \dfrac{3}{9} - \dfrac{3}{7}}}{{1,2 + \dfrac{4}{9} - \dfrac{4}{7}}}\\
= \dfrac{{\dfrac{9}{{10}} + \dfrac{3}{9} - \dfrac{3}{7}}}{{\dfrac{{12}}{{10}} + \dfrac{4}{9} - \dfrac{4}{7}}}\\
= \dfrac{{3.\left( {\dfrac{3}{{10}} + \dfrac{1}{9} - \dfrac{1}{7}} \right)}}{{4.\left( {\dfrac{3}{{10}} + \dfrac{1}{9} - \dfrac{1}{7}} \right)}}\\
= \dfrac{3}{4}\\
\Rightarrow {b^2} = {\left( {\dfrac{3}{4}} \right)^2} = \dfrac{9}{{16}}
\end{array}$