I’m not sure what your table looks like, but this is an easy way to find any missing x or y values.
If you’re given an x, plug that x into the function, for example if the x is 1, plug 1 instead of the x in the function to get 1 + 1 = 2, so the corresponding y value is 2.
If you’re given a y, plug that y into the function, for example if the y is 2, plug that 2 instead of the F(x) in the function to get 2 = x + 1 and solve for x by subtracting 1 from both sides to get 1 = x, so the corresponding x value is 1.
You can also graph the function and see which points exist.