Click Tangent Arc 
on the Sketch toolbar, or right-click in the graphics
area and select Tangent Arc. Create
a tangent arc as shown. Then
press Esc to deselect
the tool.
Hold down Ctrl, and select the centerline, both horizontal lines, and the 3-point arc.
Click Mirror Entities 
on the Sketch toolbar.
The sketch entities are mirrored across the centerline.
Dimension the oblong as shown.
The sketch is fully defined.
