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The solution and the answers by  @mananth  were   W R O N G.
3x + 2y =  69,    (1)
5x + 6y = 133.    (2)
Apply the Elimination method.  For it, multiply eq(1) by 3.  Keep eq(2) as is:
9x + 6y = 207,    (1')
5x + 6y = 133.    (2')
Subtract eq(2') from eq(1'). You will get
9x - 5x = 207 - 133 
4x =  74.  ====>  x = 
 =  18.5.
Then from eq(1)  3*18.5 + 2y = 69  ====>  2y = 69 - 3*18.5 = 13.5  ====>  y = 
 = 6.75.
Answer.  Large box weights  18.5 kg.   Small box weights 6.75 kg.
Check.   3*18.5 + 2*6.75 = 69.      ! Correct !
         5*18.5 + 6*6.75 = 133.     ! Correct !
Solved correctly.