You need to know and have memorized metric unit conversions. They are easy, you just need to know how many zeros to add.
1 cm = 10 mm
1 m = 1000 mm
1 m = 100 cm
1 km = 1000 m
(1.) 4m + 3cm = 400cm + 3cm = 403cm
(2.) 3km + 8m = 3000m + 8m = 3008m
(3.) 7cm + 9mm = 70mm + 9mm = 79mm
(4.) 1m + 5cm = 100cm + 5cm = 105cm
(5.) 5km + 5m = 5000m + 5m = 5005m
Do you see that to switch 5km to smaller unit meters you just add the zeros on to the end. Because 1km = 1000m ; 5km = 5000m
You try the rest. let me know if you have any questions.