The slope of a line that is parallel to the line whose equation is ax + by = c is D. ba. In a linear equation of the form ax + by = c, the slope can be determined by rearranging the equation into the slope-intercept form y = mx + b, where m represents the slope. Since the coefficients of x and y are a and b, respectively, the slope of the line is given by -a/b. Therefore, a line parallel to this line would have the same slope, which is -a/b or ba.