There are two unknown constants in your formula, a and b. You need two equations to solve for two unknowns so pick two points (typically, it is best to endpoints, here 1954 and 1994).
That will give you [tex]132459= ab^{1954}[/tex] and [tex]514013= ab^{1994}[/tex]
It should be easy to see that dividing one equation by the other will eliminate a, leaving a single equation to solve for b.
