whatever the amounts of "x" and "y" are, they must add up to 5Liters
thus x + y = 5
and whatever the concentrated quantity is in each, they must add up to (5)(0.14)
notice, that we use the decimal notation for the amount of juice concentration, that is, 15% is just 15/100 or 0.15, and 14% is just 14/100 or 0.14 and so on, recall that "whatever% of something" is just (whatever/100)*something
thus
solve for "x", to see how much of the 15% juice will be needed
what about "y"? well, y = 5 - x