Answer: 2
A function is basically a collection of points. Often those points are plotted and a curve is drawn through them to make a graph (eg: parabola). Each point is in the form (x,y) where x is the input and y is the output. We can say that y = f(x) because f(x) is also another way to state the output.
We want to find the output when the input is x = -3. Look through the given points to find which has an x coordinate of -3. That is the point (x,y) = (-3,2). The y coordinate here is y = 2. So that explains how f(-3) = 2