[tex]a).\;\frac{3}{5}=\frac{x}{10} \;\;\rightarrow\;\;x=\frac{3*10}{5}=6\;;\\
b).\;\frac{7}{x}=\frac{21}{15}\;\rightarrow\;x=\frac{15*7}{21}=5\;;\\
c).\;\frac{x+2}{x+7}=\frac{3x+6}{48}\;\\
\rightarrow\;\frac{48(x+2)}{(x+7)(3x+6)}=\\
\frac{48(x\not+2)}{3(x\not+2)(x+7)}=\frac{16}{x+7}\\
[/tex]