Since Ax + By = C, By = -Ax + C. So, y = (-A/B)x + C/B.
This line has slope -A/B.
Any line perpendicular to the line with equation Ax + By = C has slope -1 / (-A/B) = B / A.
Equation of line with slope B / A and containing the point (2,11) is:
y - 11 = (B/A)(x - 2)