Louisa needs 1.2 liters or 1200 milliliters of lemonade/punch for the picnic.
In a liter, there are 1000 milliliters
In 3 liters, there are 3000 milliliters
Therefore, we can set up an equation to represent how much milliliters Louisa needs.

Solve the equation and you should get 1200 milliliters.*
*If you need the number in liters, divide 1200 by 1000 and you should get 1.2 liters