The (-4,-6) are coordinates like (X,Y) now the formula your using is y= ax+b. So the only thing you have to do is fill X and y in from the coordinates into the formula.
The line shown is a horizontal line through the y-axis and has the equation y=4. A parallel line to it is a line that does not cross it. It will also have the form y=a where a is the y-coordinate of its points. The point (-4,-6) is on the line so y=-6 is the line.
