When it says f(x) and x is replaced with a number, it just means replace the x in the equation with that number.
(Simpler) Way to solve using the graph: Go to where x = -4 (4 spaces rightof the center) and look above and below it for the line. Wherever the line is, is the answer. So f(-4) = 2 because when x is -4, the y value is 2.
Example of solving with an equation: f(3) = 2 f(3) = 2(3) f(3) = 6
You create an equation for the line, than substitute -4 in to the equation.
