We have the graph of #y=3#. The slope of this line is 0 and the y-intercept is 3. The line is also perpendicular to the x-axis. If the line we are looking for is perpendicular to #y=3# then our line is going to be perpendicular to the x-axis. A line that is perpendicular to the x-axis is #x=0#. But, our line needs to go through (3,3). So we just add 3 to get the line #x=3#.