I'm using the Dixit model...

The inverse market demand function is:

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

w=10 \,

r=30

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

I have:

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

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

=\frac{120-q_1}{4}

Look good?