Math vocabulary what does The word interval Mean

12. the amount of money in a bank account decreased by 21.5% over the last year. if the amount of money at the beginning of the year is represented by ‘n', write 3 equivalent expressions to represent the amount of money in the bank account after the decrease?

Can someone check if i did this right since i really want to make sure it’s correct. if you do you so much

Over which interval is the graph of f(x) = x2 + 5x + 6 increasing?