Population density=number of mice / number of square meters.
1) we calculate the number of square meters.
area of a rectangle=length x width
area of a vegetagle garden=12 m * 7 m=84 m²
2) we calculate the population density
Population density=168 mice/ 84 m²=2 mice/m²
Answer: the population desity of the mice is 2 mice/m²