help with a simple math formula

Hello, I'm doing a bit of chemistry right now, and I need to make an autocalculator of required amount of solute (the thing that dissolves in solution) per solution (water).
I know that I need 3g of solute per 350ml of water (solution). Knowing that, how can I calculate how much of solute do I need to add to let's say 20ml of water? or 1000ml of water? I basically need to dissolve a specific amount of solute in solution, but I can't figure out a formula that could calculate that for me. Math was never my strong suit, if you have any ideas, could you please help me on this?Screenshot 2021-12-16 161513.png