To determine how much you earn for washing 7 cars, we first need to find the rate you earn per car.
You earned $64 for washing 8 cars, so:
\[ \text{Earnings per car} = \frac{64}{8} = 8 \text{ dollars per car} \]
Now, to find out how much you earn for washing 7 cars:
\[ \text{Earnings for 7 cars} = 7 \times 8 = 56 \text{ dollars} \]
Therefore, you earn $56 for washing 7 cars.