Need explanation and why each steps is done. Thank you!

2 big jars contain 0.3 kg more sugar 

than 3 small jars

     2b = 3s + .3

       re-arrange to   s =   (2b-.3)/3

5b + 2s = 8.35      total    now substitute in the red equation for 's'

5b  + 2(2b-.3)/3  = 8.35

b = 1.35 kg   per jar          now multiply by 5  (since there are 5 big jars).......

0.3÷2 = 0.15
1S : 1B = 2x  : 3u+0.15

2S+5B = 4x+15x+5×0.15 = 8.35
x  = (8.35-5×0.15)÷(4+15) = 0.4

All big →
5 × (3u+0.15)
= 5 × ( 3×0.4 + 0.15 )
= 6.75 kg (Ans)