To find the value of \( y \) when \( x = -1.5 \) using the greatest integer function (also known as the floor function), we need to calculate \( y = \lfloor -1.5 \rfloor \).
The greatest integer function \( \lfloor x \rfloor \) outputs the largest integer that is less than or equal to \( x \).
For \( x = -1.5 \), the greatest integer less than or equal to -1.5 is -2.
Therefore, \( y = \lfloor -1.5 \rfloor = -2 \).
So the value of \( y \) when \( x = -1.5 \) is -2.