Greg's favorite activities are bike riding and hip-hop dancing. He wants to try to do these activities for 6 hours each week. He would also like to try to burn off about 2,100 calories a week. He finds that biking uses about 425 calories each hour and hip-hop uses about 325 calories each hour. He wants to do bike riding for x hours and hip-hop dancing for y hours each week. How much time should he spend doing each activity to accomplish his goal?