Hello friend.here is the solution
Weight of third pizza to be considered 'x'
then, TheWeight of First one Pizza piece = x +4 
Then second pizza piece = first piece + Weight less /more 
= (x +4)+ 7 
= x + 11
Given Total weight = 300 g 
x + x + 4 + x + 11 = 300 
3x = 300-15 
x = 285/3 
x = 95 
Therefore, Weight of 
1) 1st Piece = x +4 = 95 +4 = 99 
2) 2 nd Piece = x + 11 = 95 +11 = 106 
3) 3 rd piece = x = 95 
hope this solution for your question helps you