Since 120° is 180°-60°, we first draw a 60°.
To do that we draw equilateral triangle QTR
with each side equal to 5.6cm,since its right
side will be QR which is 5.6cm.

We begin by drawing a line of arbitrary length, label the
left endpoint T.  Use the ruler to find point R so that
TR is 5.6cm long. 



Open your compass to the distance 5.6cm. Put the sharp point
of your compass on T and swing an arc, like this red one:



Keeping your compass open to the distance 5.6cm, put the sharp point
of your compass on R and swing an arc, intersecting the first arc,
like this, labeling the point Q where the two arcs intersect:



Draw QR.  You can draw QT also if you like, to show that triangle QTR
is an equilateral, and therefore the angle QRT is 60°.



Use the ruler to find point S so that RS is 7.8cm long.  That's 
because opposite sides of a parallelogram are equal and since the
upper side PQ is going to be 7.8cm, side RS must also be 7.8cm.




Keeping your compass open to the distance 5.6cm, put the sharp 
point on S and find point U between R and S so that US=5.6cm.



Next we duplicate what we did at T and R at U and S,
and label the point where the arcs intersect as P.




Draw SP and PQ and you're done:



Edwin