a. Here is the game tree representing the situation:
```
Firm 1
/ \
/ \
Downtown Uptown
/ \ / \
/ \ / \
Downtown Uptown Downtown Uptown
```
b. The Nash equilibrium of this game is for both firms to locate in Uptown. If Firm 1 locates in Downtown, Firm 2 has an incentive to locate in Uptown, since it earns a higher profit of $1,300,000. Similarly, if Firm 1 locates in Uptown, Firm 2 has an incentive to locate in Uptown, since it earns a higher profit of $700,000. Therefore, the only stable outcome is for both firms to locate in Uptown, where they both earn a profit of $700,000.