Final answer:
The equation of the line with a slope of 3 and a y-intercept of 9 is y = 3x + 9.
Step-by-step explanation:
The equation of a line with a given slope and y-intercept can be found using the slope-intercept form, which is y = mx + b. In this case, the slope (m) is given as 3, indicating that the line rises 3 units for every 1 unit it moves horizontally. The y-intercept (b) is given as 9, meaning that the line crosses the y-axis at the point (0, 9). Now, inserting these values into the slope-intercept form yields the final equation of the line: y = 3x + 9.