Solution
= Solution
Reduction of both coefficients in <modular arithmetic> gives
$$
754=8\cdot91+26,\qquad -507=-6\cdot91+39.
$$
More explicitly,
$$
754x+507=91(8x+6)+(26x-39).
$$
The first expression is divisible by $91$ exactly when the last parenthesis is. By the definition of a <integer congruence>, \b[the two congruences are equivalent].