Answer: 1


Using proportion
 

4.5 litres of antifreeze in new mixture
1 more litre needed


Using algebra
Let the extra volume of antifreeze required be $x$ litres. Initially, there are 3.5 litres of antifreeze in the solution. So $3.5+x=0.3(14+x)$, that is, $35+10x=42+3x$, so $x=1$.