Let us say that:
 x = mass from Earl Grey Tea
 y = mass from Orange Pekoe Tea
  
 So that:
 x + y = 600
 and that:
 6 x + 4 y = 4.5 * 600
 6 (600 – y) + 4 y = 2700
 3600 – 6y + 4y = 2700
 -2y = -900
 y = 450 pounds
 x = 600 – 450 = 150 pounds
  
 Answer:
  Earl Grey Tea = 150 pounds
 Orange Pekoe Tea = 450 pounds