If u = {natural numbers less than 20} and n = {factors of 18}, what is n

This is from the answer key to this question;

N= {4, 5, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 19.}

1 number

if we use a graphing calculator, there is only one such number that satisfies the conclusion of the mean value theorem, that is, a number such that

where the right-hand side is the average rate of change of f on the interval [0.5,0.75], which is the slope of the secant line.

in the attached graph, the black line is the secant line which has its slope as the average rate of change of f on [0.5,0.75], while the orange line is the only tangent line to the graph of f in the interval that satisfies the mvt conclusion.