Linear function only?

Okay.

Are you familiar with y = mx +b ?

That is the slope-intercept form of the equation of a line.

Adapting that to your question here,

q(t) = 290 +40*t ----------answer.

Where

q(t) is "q of t", is q as a function of t, is in million lbs.

t is in years, based from year 1990.

-----------------

When t = 0, or in 1990,

q(0) = 290 +40*0

q(0) = 290 million lbs. -------this is given.

In year 2005,

t = 2005 -1990 = 15 years

So,

q(15) = 290 +40(15)

q(15) = 290 +600

q(15) = 890 million lbs --------answer.