Answer:
The length of the shelf is 129 cm or 1290 mm.
Explanation:
Since Samuel will use the two boards to make the shelf, then the total length of the shelf is the sum of the lengths of each board. So in order to solve this problem we must do as shown bellow:
length shelf = length board1 + length board2
length shelf = 84 + 45 = 129 cm
In order to convert this to milimeters we must multiply the answer above by 10. So we have:
length shelf = 129*10 = 1290 mm
The length of the shelf is 129 cm or 1290 mm.