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