9.
P(x) = 55000√[ x - 1945 ]
In which year was the population of the city 220,000 ? That is, what is x when P(x) = 220,000 ?
220,000 = 55000√[ x - 1945 ]
Divide both sides of the equation by 55000
4 = √[ x - 1945 ]
Square both sides of the equation.
16 = x - 1945
Add 1945 to both sides.
1961 = x
Check: P(1961) = 55000√[ 1961 - 1945 ] = 55000√[ 16 ] = 55000(4) = 220,000