Answer :
[tex]\frac{3}{4b}+9=21[/tex]
subtract 9 from both-side
[tex]\frac{3}{4b}=21-9[/tex][tex]\frac{3}{4b}=12[/tex]cross multiply
[tex]48b=3[/tex]Divide both-side by 48
[tex]b=\frac{3}{48}[/tex][tex]b=\frac{1}{16}[/tex]