We have the next convertions
For the first one
10mm=1cm
therefore
780/10=78
780mm=78cm
For the second one
10000cm^2=1m^2
1500 cm^2 = 0.15m^2
for the third one
1m^2=1000000mm^2
0.9 m^2 = 900000 mm^2
for the fourth
10000cm^2=1m^2
6.3 cm ^2 = 0.00063 m^2