For this problem, I used the Doppler Effect equation, and got 419.361 as my answer. I used + on the top and - on the bottom. Do I need to switch the two?

Since the suspect is going in the direction that is away from the sound source, you use a negative value for the observer speed. The source is moving in the direction toward the observer, so there is a positive source speed. That means you get (using the equation correctly):
f_observed = f*[v+(-vo)]/[v-(+vs)]

so there ends up being a - on both top and bottom. Did you follow that?