What additional steps can be taken to mitigate the threat in the event that terrorists were able to acquire WMD's and actually make an attack?
First of all, it is impossible to say what “additional steps can be taken” when we do not know what steps you have already been told about in your class or in your textbook. The only thing that we can do is to list steps that could be taken. That is what I will do in this answer.
In this question, you are asking about steps that can be taken to mitigate that harm that is done if terrorists acquire WMDs and actually carry out an attack of some sort, presumably on American soil. I would say that most of the steps that can be taken are steps that should be taken, or at least planned out, well before any attack actually occurs. There are some steps that could be taken after the attack, but the more that can be planned before any attack, the better.
One step that can be taken before hand is to “harden” targets that would cause the most damage if attacked. For example, let us imagine that there is going to be a major sporting event with 100,000 or more people in attendance. One way to mitigate the impact of an attack on such an event would be to keep cars relatively far away from the venue in which the attack is occurring. Another way would be to screen people before they get near to the venue. In that way, a terrorist with (for example) a dirty bomb would be unable to detonate it in the venue itself but would be forced instead to detonate it farther away where fewer people might be affected. Similarly, we might make it easier to cut off water supplies from a given reservoir so that a chemical weapon introduced into that reservoir could be prevented from getting into the overall water system of a given city. In ways like these, we can try to make it harder for attacks to have a very large impact.
A second step that could be taken is to anticipate the types of attacks that might happen and plan responses to them. If governmental agencies have contingency plans in place before an attack happens, they can respond to the attack more quickly and with less confusion than if they have no such plans. A quick and orderly response will mean that help can reach victims of the attack quickly and the casualties can be minimized.
Finally, it would be good to educate the public as much as possible. If the public can be educated about how to act in the event of various sorts of attacks, mass confusion might be prevented if an attack actually occurs. If people can be educated, for example, about the sorts of supplies they might need to have on hand to cope with an attack, an actual attack would find fewer people unprepared. If people were prepared for the attack its aftermath would have less of an effect on them.
All of these are possible steps that might be taken to mitigate the impact of a terrorist WMD attack.