That formula relies on the sample proportion, the standard deviation of the sample proportion, and the appropriate z-score for a 90 percent confidence interval.
Here are some hints for finding those.
The sample proportion is the share of people who enjoy hard rock music. It's usually denoted as p with a "hat" or carrot or half-arrow above it.
The standard deviation is given by a formula that you will need to find. That formula is usually written with an n, which denotes the sample size. What is the sample size in your case?
The z-score can be found on a table of z-score values. There should be one in the appendix of your statistics textbook. You can also find one online.
Quickly, here is how a z-score table works: Go down to -2.9 in the left-most column, then move your finger to the right until it is under 0.07; the entry is 0.0015. This means that 0.15% of the area of the distribution is to the left of z = -2.97.
A 90 percent confidence interval corresponds having 90 percent of the distribution between your z's. That means 10% of the distribution will not be between the z's. 5% on the left and 5% on the right.
What z will get you 5% on the left? What z will leave you 5% on the right?
I hope this helps! Good luck.