You can put this solution on YOUR website!
if I have a 12% acid solution and a 21% acid solution and i need a 8% acid solution with 101 ML how would I solve this in an algebraic equation
This isn't possible, you are wanting a lower percent solution than either of those available.
One has to be above what you want, and one has to be below what you want.
In this case you could mix one of the solutions with pure water and get 8% if you want to.
Let me know if you want to do it this way.