Final answer:
The equation of the straight line with a slope of 3 and a y-intercept of 9 is y = 3x + 9.
Step-by-step explanation:
The equation of the straight line can be determined using its slope and y-intercept. According to the information provided, the slope (m) is 3, which means there is a rise of 3 on the y-axis for every increase of 1 on the x-axis. The y-intercept (b or c) is the point where the line intersects the y-axis, and that value is given as 9. Thus, the equation of the line in the form y = mx + c is y = 3x + 9.