Final answer:
The function rule of f(x) = 18x - 7 is a linear equation with a slope of 18 and y-intercept of -7. The function represents a straight line within the defined domain of 0 ≤ x ≤ 20, indicating a consistently increasing function within that interval. None of the provided choices matches a function with a positive, decreasing slope.
Step-by-step explanation:
The question asks for the function rule of f(x) = 18x - 7. This expression is a linear equation, representing a line with a slope (rate of change) of 18 and a y-intercept (the point where the line crosses the vertical axis) of -7. In the context of the provided information on various function rules, the key identifiers are the slope and the y-intercept when determining that f(x) = 18x - 7 is a linear function.
For example, considering f(x) in a range from 0 to 20, we understand that x represents a real number within that interval. If this function plotted as a graph would produce a straight line that ascends from left to right, because the slope is positive.
In regards to the given choices where a function has a positive value and positive slope that decreases in magnitude, neither functions y = 13x (which has a constant slope) nor y = x² (which has an increasing slope), perfectly match the description. However, since f(x) is generally decreasing in slope, we might be discussing a function with a lessening rate of change, not covered by the provided choices.