Answer:
Step-by-step explanation:
Step 1:
Click line tool (option given on tool palette labelled as Pfloor).
Step 2:
To plot the line, left click your mouse button on any vertical point greater than 50 for plotting first point, then move your cursor horizontally to the right side and click again to plot the second point.
This should give you a line intersecting both S1 and D1 lines and also having height greater than 50 as measured on the vertical axis.