When writing functions, f(x) is essentially y. So solve for y by subtracting x from both sides. Your new equation is y=-x+14. Now, make a table of various x values (such as 0, 2, 4... etc) and a y column. Input the given x value into the x column and the equation y=-x+14 and solve for y. Now, put the solution for y into your y column. To show your work on one place, you could put an extra column in between x and y, and label it y=-x+14, where you will solve for y within the column, if that makes sense. Hopefully this can be useful in the future :)