không có gì'

\(15:x=5\)

\(x=15:5\)
\(x=5\)

\(x:4=3\)

\(x=3.4\)

\(x=12\)

\(21:x=7\)

\(x=21:7\)

\(x=3\)

\(=-\dfrac{13}{7}-\dfrac{4}{9}+\dfrac{10}{7}+\dfrac{4}{9}=\left(-\dfrac{13}{7}+\dfrac{10}{7}\right)+\left(-\dfrac{4}{9}+\dfrac{4}{9}\right)=-\dfrac{3}{7}+0=-\dfrac{3}{7}\)

diện tích HBH ABCD là

\(3.5=15\left(m^2\right)\)