x=0 , y=2x-5 and y=mx+9 form a right angle. So, y=mx+9 is either perpendicular to x=0 or y=2x-5 because the angle between x=0 and y=2x-5 is not 90 degrees. The product of slopes of perpendicular lines equals to -1.
Thus, m=-1/2 if it is perpendicular to y=2x-5. And y=c, for some constant if it is perpendicular to x=0. So y=0x+9 fits the bill. (m=0, in this case)
