Final answer:
In the given linear equation y = 9 + 3x, substituting x = 9 yields y = 36, showing that y is directly proportional to x with a constant of proportionality equal to 3.
Step-by-step explanation:
The problem given to us is a linear equation in the form y = 9 + 3x. This equation represents a straight line where the y-intercept is 9 (that is the value of y when x is 0) and the slope (m) is 3, indicating that y increases by 3 units for every 1 unit increase in x. To find the value of y when x equals 9, we substitute 9 for x in the equation, resulting in y = 9 + 3(9). This simplifies to y = 9 + 27, which then further simplifies to y = 36. Therefore, when x = 9, the value of y is 36.