7n-5n=6 how do we solve this!

Be quick please also I need to show work

Olivia uses the work below to determine 55% of 720 which explains the error in olivia’s solution

Assume that there is a 11% rate of disk drive failure in a year. a. if all your computer data is stored on a hard disk drive with a copy stored on a second hard disk drive, what is the probability that during a year, you can avoid catastrophe with at least one working drive? b. if copies of all your computer data are stored on four independent hard disk drives, what is the probability that during a year, you can avoid catastrophe with at least one working drive?

The value of cos 30° is the same as which of the following?