Estimate the value of the function x=-1

Explanation:

You draw a vertical line going upward from x = -1, stop where your vertical line intercetpts the function (the red line); then draw a horizontal line unitl you reach the y-axis; finally, you can read the y-value on the y-axis. That is the value of the function.

It is pretty close to the midpoint between y = 1 and y = 2, thus a good estimate would be y = 1.5.