You still haven't mentioned the radius of the arcs -- the inside and outside radius of the bend itself. Or is one of those the 76 mm measurement?
It appears that the 75 mm measurement doesn't matter.
Here is a picture that shows all the pieces you need:
You have AB = 60 mm; DF = CE = 20 mm; and radius AC. You want to find FG.
One way would be to make a coordinate system and write the equation of the circle through A. Another is to use the Pythagorean Theorem on right triangle CEG (where you know radius CG and length CE) to find EG; then subtract EF (= CD) from that.