An equation of a line in slope-intercept form is y = mx + b, where m is the slope and b is the y-intercept. Since the line passes through the points (-6, 3) and (-6, 6), we can see that the x-coordinate is the same for both points. This means that the line is a vertical line, and has an undefined slope (division by zero).
The equation of a vertical line is x = constant, where the constant is the x-coordinate of the line. In this case, the constant is -6, so the equation of the line is:
x = -6