What is the format of this proof?

The equation x2 – 1x – 90 = 0 has solutions {a, b}. what is a + b?

Calculate the missing value. round the answer to on decimal place. start with 70, increase it by 21%, and end up with

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?