One can write a linear equation by setting up 2 ordered pairs (t, P). Then calculating the slope and then finding the equation.

at 1990 let t =0, therefore 1992 will be t =2, and 1996 t = 6

the ordered pairs (t,P) are (2, 3610) and (6, 4290)

the formula for slope is (P - P)/(t -t), (4290-3610)/ (6-2), which simplifies to 680/4 which is 170

using point slope form of the equation. P - 3610 = 170(t - 2)

this can be rearranged to P = 170t + 3270