This is a word problem, and not all of the words are relevant. Let's find the ones that are.
Susie used 2/5 of a liter of water to make a squash, and all we need to do is find out what 3/7 of that amount of water is. We can ignore the squash entirely. The question is: What is 3/7 of 2/5 liters?
Any time you're working with fractions, "of" means "multiply."
To multiply fractions, we multiply the numerators together and the denominators together, to find the solution's numerator and denominator.
The new numerator is 2*3, which equals 6.
The new denominator is 5*7, which equals 35.
Next, we double check that 6/35 is in simplest form. The factors of 6 are 1, 2, 3, and 6. The factors of 35 are 1, 5, 7, and 35. With no factors in common other than 1, 6/35 can't be simplified. It is in simplest form!
3/7 of 2/5 liters of water is 6/35 liters of water.