Suppose 6 quarts of a solution that is 45% antifreeze is mixed with 2 quarts of a solution that is 19% antifreeze.

6+2=8 quarts of a new solution

0.45*6 - quarts antifreeze in a solution that is 45% antifreeze

0.19*2 - quarts antifreeze in a solution that is 19% antifreeze

x - concentration of a new solution

8x - quarts antifreeze in a new solution

0.45*6 + 0.19*2 = 8x

3.08=8x

x = 0.385 = 38.5 %