Let x be the amount in litres of the 100% alcohol (pure alcohol).
10% of 7 litres is 0.7L alcohol. There are x litres of pure alcohol.
The total quantity of the mixture is 7+x litres.
The amount of alcohol is 0.7+x=0.3(7+x) because the mixture contains 30% alcohol.
So 0.7+x=2.1+0.3x, 0.7x=2.1-0.7=1.4, x=1.4/0.7=14/7=2L.
2 litres of pure alcohol are needed to make a 30% alcohol mixture.