f(x) means y!
What is the equation of a line?  y = mx + b where m is slope and b is the y-intercept.  So here the slope is -2 which means as x increases, y decreases.  Look at the 4 tables: only one of them is decreasing when x is increasing.
We need to look carefully because the x values skip from 0 to 2 to 4 to 6.
Let’s check this with the graphing calculator.  Two more points 🙂