f(x)= y     f(x)=y
f(0)=-5 and f(3)=4 
 (0, -5)     (3, 4) 
So the function f(x) goes through the two points:

(x,y) = (0,-5) and (x,y) = (3,4)

Plot those two points:



f(x) could be a straight line through them, like this



That has the equation f(x) = 3x-5, which is found by substituting
the two points into the slope formula to find the slope,
and then substituting in the point-slope formula, and
simplifying.  Then when you have y = 3x-5, substitute f(x) for y.

Or it could be this function



which is 

Or it could be this function



which is 

Or it could be any of thousands of other functions.

Edwin