`P(x) = 80000(x-1940)^(1/3)`

P(x) is the population and x is the year which is between 1955 and 1995.

When population is 290000 then `P(x) = 290000`

`P(x) = 80000(x-1940)^(1/3)`

`290000 = 80000(x-1940)^(1/3)`

`290000/80000 = (x-1940)^(1/3)`

`3.625 = (x-1940)^(1/3)`

`3.625^3 = (x-1940)`

`x = 1987.6`

*So the population of the city will be 290000 in the year of 1987.*