[tex]a) \frac{x}{28} = \frac{3}{7} =\ \textgreater \ x= \frac{28*3}{7} =12[/tex]
[tex]b) \frac{45}{x} = \frac{3}{7} =\ \textgreater \ x= \frac{45*7}{3} =105[/tex]
[tex]c) \frac{2x+1}{21} = \frac{3}{7} =\ \textgreater \ 7(2x+1)=21*3;14x+7=63=\ \textgreater \ 14x=56[/tex] =>x=4