$2 a+10-4 x=-4 x+21$  $2 a=11 $  $a=\frac{11}{2}$ 
What value of $a$ would make the solution to the equation, $$ 2 a+3-4 x+7=3\left(-\frac{4}{3} x+7\right) $$,~"infinitely many solutions"?~\uline{~~~~~~~~~~}~ 
Options:
  * $-5$ 
  * $\frac{11}{2}$ 
  * $10.5$ 
  * $$21$$ 