
The slope of a perpendicular line is the opposite reciprocal of the given line's slope.
The slope of the green line is
.
First, find the opposite of this.

Then, find its reciprocal by flipping the numerator and the denominator.

This means the slope of the red line is
.
Note: You forgot to attach the graph of the shown lines, but given the slope of one line, I was able to find the slope of the other, knowing that the two lines are perpendicular, which makes an X formation.