You need 845 ml of a 75% alcohol solution. on hand, you have a 35% alcohol mixture. how much of the 35% alcohol mixture and pure alcohol will you need to obtain the desired solution?
you will need
ml of the 35% solution
and
ml of pure alcohol.

Let be the amount (mL) of the 35% solution you need to use, and the amount (mL) of pure alcohol. Mixed together, you want to end up with 845 mL of solution, so that

For each mL of the 35% solution used, there is a contribution of 0.35 mL of alcohol, while each mL of the pure alcohol solution contributes 1 mL of alcohol. You want to end up with a solution at 75% concentration, so that

We can solve easily by substitution:

Then

So you will need 325 mL of the 35% solution and 520 mL of pure alcohol.

3 3 6, because in every triangle the longer side is always shorter than the sum of the other two