Note that it was not necessary to compute the last two entries -365 and 1876 in(*). It is a good idea however to check that equation (**) holds. In this case we have:
136*1876 = 255136
(-699)*365 = -255135
1
So it is correct
Why Blankinship’s Method works: Note that just looking at what happens in the first column you see that we are just doing the Euclidean Algorithm, so when one element in column 1 is 0 , the other is, in fact, the gcd. Note that at the start we have
[]
And
A = 1*a+0*b
B = 0*a +1*b
One can show that at every intermediate step
[]
We always have
A1 = x1a + x2b
B1 = y1a + y2b
And the result follows. I will omit the details.