.75(x+3)=.5(3)+x
You already have 3 liters, and you are going to add x liters.
50% (.5) of the 3 liters is acid, and 100% (1) of the added x liters is acid. You will end up with (3+x) liters.
If you have been assigned this problem, you should be able to solve this equation...
Hope this helps you solve this, and remember how to do others like it.