y=5, x=4 on a graph is a line straight up on the 4th point on the x-axis. a perpendicular line would be on its side going through the straight line. if u plot (3,5) and draw a line on its side going through the straight line of x=4, then u would have done what they asked. the answer would just be the y intercept of the line for (3,5) which is y=5. if u plot y=5, it is on its side going through x=4.