You always want to try and identify the quantities you're being asked to find and give them letters. In this case, you're being asked for the number of millilitres of solution 1: call this X; and the number of millilitres of solution 2: call this Y. One piece of data is that the total, X+Y, is 600. You now have to use the rest of the data to get another relationship between X and Y. The amount of sodium in the mixture is 15% of X plus 30% of Y, which is 15X/100 + 30Y/100. You're told that this is equal to 22% of the total, that is 22% of X+Y or 22(X+Y)/100. So we have two equations, X+Y = 600 and 15X/100 + 30Y/100 = 22(X+Y)/100. Solve as a pair.