I just solved the previous problem going through all the steps. This one I'll stop short so that you can use your critical thinking skills to complete. The steps are essentially the same, after all.
Let M = Mercury Building and H = Howard building. Here's what we know:
M = 2H (Mercury is twice as tall as Howard)
M + H = 99 (their total number of stories equals 99 floors)
We have a a value for M, so plug it in:
2H + H = 99
3H = 99
You should be able to solve for H from here, and then you can use that value to solve for M.