Im confused, not sure how to do this. me it is geometry, specifically perpendicular lines with an equation and a point

The line y = -3 is a horizontal line. The perpendicular line will be vertical, so will have an equation of the form
  x = c
for some constant c.

You want the vertical line to go through the point (4, -6), so the required value of c is the x-value of that point: 4

Your equation is
  x = 4