The blue line is 80(given) and each red line is 2(also given) so we know that the pink line 76(80-2-2=76) ; know what about those two black line? we need to know those to find the green line!

The blue line is 80(given) and each red line is 2(also given) so we know that the pink line 76(80-2-2=76) ; know what about those two black line? we need to know those to find the green line!