Z =x^2-y^2 for find the domain​

This is so hard and it’s for a test

Lin is writing an equation to model the proportional relationship between y, the total cost in dollars of downloading videos from a website, and x, the number of videos downloaded. she knows that the total cost to download 3 videos was $12. her work to find the equation is shown below. joylin’s work step 1 k= 3/12= 0.25 step 2 y= 0.25x where did joylin make her first error?

How many sides does a regular polygon have if each exterior angle measure is 45 degrees explain your answer, i really don't understand this.