Area of the larger square = 12² = 144 cm²
Area of the smaller square = 7² = 49 cm²
Area of the section = 144-49 = 95 cm²
18900cm = 189m
189 / 1.8 = 105m.
The blackboard's length in meters is 105m.
1.7 times 10 to the 9 power
1.52 times10 to the 8 power