For parallel lines ax + by + c1 = 0 and ax + by + c2 = 0 (same coefficients of x and y), the distance is |c1 - c2| / sqrt($a^2$ + $b^2$). Important: the coefficients of x and y must be identical (not just proportional) before applying this formula. If given 2x + 3y = 5 and 4x + 6y = 7, first rewrite the second as 2x + 3y = 3.5, then d = |5 - 3.5| / sqrt(4 + 9) = 1.5/sqrt(13).