Answer:
once you have f(x) = 3-2x,
f(0) means the value of 3-2x after substituting 0 for x, giving f(0)=3.
This means the point (0,3) is on the graph of f(x). That point is on the y axis.
Since a plot of f(x) intercepts the y axis at f(0), the value of f(0) is called the y intercept.