To find out how much you earn for washing 3 cars, first determine your earnings per car.
You earn $49 for washing 7 cars, so:
\[ \text{Earnings per car} = \frac{49}{7} = 7 \text{ dollars per car} \]
Now, multiply the earnings per car by the number of cars you wash:
\[ \text{Earnings for 3 cars} = 7 \times 3 = 21 \text{ dollars} \]
Thus, you earn $21 for washing 3 cars.