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?