I'm using the Dixit model...

The inverse market demand function is:

$\displaystyle p=100- \frac{q}{2}$

$\displaystyle w=10 \,$

$\displaystyle r=30$

To start I have to find firm 2's (the entrant) best response function after it has decided to enter.

I have:

$\displaystyle q_2=\frac{100-\frac{q_1}{2}-(w+r)}{2}$

$\displaystyle =\frac{200-80-q_1}{4}$

$\displaystyle =\frac{120-q_1}{4}$

Look good?