trigger other earthquakes - when there is a large magnitude earthquake it lead to shake the earth's surface. Thus it can create more earthquakes on the same place or neighboring places.
cause billions of dollars - when there is a large disturbance it can destroy big buildings, homes, etc. Which is basically a billion dollar loss.
cause tsunami - If there is a disturbance in the earth's surface near sea's, etc there is chance of tsunami.
To solve for c, you divide both sides by -18. If you do that, your answer will be -1/16. So, I think you typed the question wrong. Pretty sure it's the first option.
