x = large boxes, y = small boxes
5x + 3y = 110 .....multiply by -2
2x + 6y = 74
------------------
-10x - 6y = - 220 (result of multiplying by -2)
2x + 6y = 74
-----------------add
-8x = - 146
x = -146/-8
x = 18.25 <== large box weighs 18.25 kg
2x + 6y = 74
2(18.25) + 6y = 74
36.50 + 6y = 74
6y = 74 - 36.50
6y = 37.50
y = 37.50/6
y = 6.25 <=== small box is 6.25 kg