Answer:
when the 2 boards are put together, the total length will be 7 m 11 cm
Explanation:
the first board has 3 m 12 cm , but since 1 m = 100 cm :
length board 1 = 3 m 12 cm = 3 * 100 cm + 12 cm = 300 cm + 12 cm = 312 cm
the same happens with the board 2
length board 2 = 3m 99 cm = 3 * 100 cm + 99 cm = 300 cm + 99 cm = 399 cm
the 2 boards together will have
total length = length board 1 + length board 2 = 312 cm + 399 cm = 711 cm
since 7 m = 700 cm
total length = 711 cm = 7 m 11 cm