Three things you first need to realize are these:

1.  "f(x)" is the same as "y".
2.  f(0)=48 is the same as "(0,48) is a point on the line of the graph of f(x)".
3.  f(2)=0 is the same as "(2,0) is a point on the line of the graph of f(x)".

So the problem is the same as:

Find the equation of the line through the two points (0,48) and (2,0).

Use the slope formula m =  with (,) = (0,48) and (,) = (2,0)


m = 
m = 
m = 
m = -24

Then we use the point-slope formula for the equation of a line,
which is 

y - y1 = m(x - x1)

y - (48) = (-24)(x - 0)
  y - 48 = -24x
       y = -24x + 48

Finally we replace "y" by "f(x)":

    f(x) = -24x + 48 
       
Edwin