The division algorithm is a theorem that says that given a dividend a and a divisor b there is a unique quotient q and remainder r satisfying a = qb + r and 0 ≤ r < b.
In your problem you have r = 43, q = 344, a = 43043 so
a = qb + r
43043 = 344b + 43
b = (43043 - 43)/344 = 125 ← Answer