At one time the Thames River in England supported an abundant community of fish. Pollution then destroyed all the fish in a 40-mile stretch near its mouth for a 45-year period beginning in 1915. Since then, improvement of sewage treatment facilities and other ecological steps have resulted in a dramatic increase in the number of different fish present. The number of species present from 1967 to 1978 can be modeled by f(x) = 6.15x - 12,059, where x is the year. 

(a) Estimate the year when the number of species first exceeded 70. 

(b) Estimate the years when the number of species was between 50 and 100.