f(x) is a fancy replacement for the typical variable 'y' that we usually see in linear functions with two variables (x & y)

When you see f(0) = -3 notice that the # in the parenthesis is basically the term 'x' in the general form f(x) and the value -3 is the value of the function when x = 0, in this case -3

Assuming you are being asked for** TWO separate linear functions** that represent f(0) = -3 and f(7) = 11

1st linear function would be f(0) = -3 can be** f(x) = x - 3**. When you plug in x = 0, y or the function = -3

Similarly, the 2nd function f(7) = 11 can be** f(x) = x + 4**. So when you plug in x = 7, y of the function = 11

Hope this helps :)