Which expressions are equivalent to -9x-21

What is the solution to the system of the equations? x = y - 3 x + 3y = 13

Officer brimberry wrote 24 tickets for traffic violations last week, but only 9 tickets this week. what is the percent decrease? give your answer to the nearest tenth of a percent.

If cos(2x) = tan^2(y), show that cos(2y) = tan^2(x).