Answer
$[1,4]$
Work Step by Step
The function remains constant on an interval when, moving from left to right, the y-values continue to remain constant for all x-values in that interval. For the graph in the question, the y-values remain constant for all values of $x$ between $1$ and $4$. Since both $1$ and $4$ are included in the interval, square brackets will be used on both sides.
Therefore, the function is increasing on the interval $[1,4]$.
