In order to estimate the size of the crowd, let's find the area of the parade route.
The length is 3 miles, that is, 3 * 5280 = 15840 ft.
The width is 2 times 21, so 42 ft.
Therefore the area is:
![A=15840\cdot42=665280](https://img.qammunity.org/2023/formulas/mathematics/college/4aa46ffn2g6zh3ynq81tuswiexdr4d445c.png)
Now, to estimate the size of the crowd, let's use a rule of three, knowing that the area for 17 people is 7 * 7 = 49 ft²:
![\begin{gathered} 17\text{ people}\to49\text{ ft}^2 \\ x\text{ people}\to665280\text{ ft}^2 \\ \\ (17)/(x)=(49)/(665280) \\ x=(665280\cdot17)/(49) \\ x=230811.43 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/hfu6u3si2bd3daeobixopf1s2iptcp5xir.png)
Rounding to the nearest integer, we have 230,881 people.