Final answer:
The equation of the straight line with a slope of 3 and y-intercept of 9 is y = 3x + 9.
Step-by-step explanation:
The equation of a straight line is generally represented as y = mx + b, where m represents the slope of the line, and b represents the y-intercept.
The equation of the straight line can be written in the form y = mx + c, where m is the slope and c is the y-intercept. The slope, which represents the change in the vertical axis over the change in the horizontal axis, is given as 3 in this example. The y-intercept, which is the point where the line intersects the y-axis, is given as 9.
The slope is defined as the rise over the run. In the given example, since the slope is indicated as a rise of 3 for every unit increase in x, we have m = 3. The y-intercept is given as 9, which is the point where the line intersects the y-axis when x = 0, thus b = 9. Putting these values together, the equation of the straight line in the example is y = 3x + 9.