x	-3	-2	1	3	4
f(x)	0	 3	12	18	21

One way is to plot the points and see what the graph looks like



Hmm!  Looks like a straight line could be drawn through all
5 of those points.  I'll get my ruler and green pen and see:



Sure looks like a straight line to me.  So I'll just find the equation
of the line that goes through, say, the first two points (any two
of the points would do).

(-3,0), (-2,3)











Then I'll substitute that in the point-slope form:









That's the answer.

Let's check to see if it works with all 5 points:

Substituting (-3,0)






Substituting (-2,3)






Substituting (1,12)






Substituting (3,18)






Substituting (4,21)






Yeppers, it works for all 5 points,
so the equation is 

y = 3x + 9

Edwin