I think the answer for "Use functional notation to express the number of graduates in 1990" if obvious:

N(1990).

Here N is the name of the function and 1990 is the value of the argument.

Now how to estimate this value. Of course we don't know it, actually it can be rather different. I think they want us to suppose that the function is linear between 1989 and 1991.

( actually, because 1990 is the midpoint of [1989, 1991], the answer is also a midpoint of [N(1989), N(1991)], i.e. (2.47 + 2.29)/ 2 = 4.76/2 = 2.38 )

So we have to write equation of this line and substitute t=1990. This equation is

(t - t1)/(t2 - t1) = (N - N1)/(N2 - N1),

where t1 = 1989, t2 = 1991, N1 = 2.47 and N2 = 2.29. So