write a linear function f with the values f (0) =4 and f (3)= -8

(0,4) and (3,-8) has segment of slope -8-4 / 3-0 = -12/3 = -4. y intercept is given to us as 4

equation is

y-4=-4(x) in point slope form
y+8=-4(x-3) using the other point
y=-4x+4 in slope intercept form
4x+y=4 in standard form