Hope this makes sense.
J=2 and I=1 from the second sum first column. As when 1 is "borrowed" from the 2 it will become 1-1 and equal 0.
Therefore G=4 from the second sum last column G-J=J becomes 4-2=2
From the first column of the first sum we know that 1 must be "borrowed" from the A as it becomes F in the answer. So A-1=F
From the second column of the first sum B-F=4 but we also know that B is a smaller number than F as it needs to borrow 1 from A.
A F possible combinations are: 6, 5 7,6 8,7 9,8
So as B-F=4. B can be 9 0 1 2
It can't be 9 as it has to be smaller than F or 1 or 2 as these are already assigned so
B=0
From there F=6 second column first sum
A=7 first column first sum
C=8 second column second sum
E=9 last column first sum
D=3