The line y = 8 is horizontal or flat because equations of the form y = a (a being a constant) are flat.
So, we know that the line they want our line to be perpendicular to is horizontal. The only kind of a line that is perpendicular to a horizontal line is a vertical line. So that means we need an equation for a line that is vertical.
Equations of the form x = a are vertical.
They told us that our line has to pass through (0, 0). Since a vertical line only has one x-coordinate for the entire line (because x is always equal to what a is), and it passes through all the other y-coordinates, we know that x has to be equal to 0.
So x = 0 would be our line.
Hopefully that was clear and helps.