Solving system by substitution 3x-4y=-10 3x-4y=-13

Two customers went to a post office. first customer paid $12 for 14 and 5 envelopes the second paid 24.80 for 10 postcards and 15 envelopes. what is cost of each envelope?

At a sale this week, a desk is being sold for $213. this is a 29% discount from the original price. what is the original price?

Which of the expressions (on the photo document i inserted) together represent all solutions to the following equation? 8cos(12x)+4=-4 *the answer should be in degrees.* > i would like for someone to explain the process to find the solutions for this.