Press Ctrl, select the edges
shown, then click Convert Entities
on the Sketch toolbar
to convert the selected model edges into sketch entities.
Click Extend
Entities 
on the Sketch toolbar, then click the converted edges.
Press Ctrl, select the edges
shown, then click Convert Entities
on the Sketch toolbar
to convert the selected model edges into sketch entities.
Click Extend
Entities on the Sketch toolbar, then click the converted edges.
Each vertical line is extended to meet the offset horizontal line.
Sketch a horizontal line to connect the two converted edges across the top.
Sketch two vertical lines, then dimension them as shown.
Click View,
Sketch Relations to hide the display
of sketch relations in the graphics area when too many relations cover
the sketch geometry.
