[tex]\frac { 3ax }{ 5 } -4c=\frac { ax }{ 5 } [/tex]
First multiply both sides by 5:
[tex]5\cdot (\frac { 3ax }{ 5 } -4c)=5\cdot \frac { ax }{ 5 } \\ 3ax-20c=ax[/tex]
then take the ax to the other side :
[tex]3ax-20-ax=0[/tex] and simplify:
[tex]2ax-20c=0[/tex] factor:
[tex]2(ax-10c)=0\\ ax-10c=0\\ ax=10c\\ x=\frac { 10c }{ a } [/tex]