The son is 12 now and the father is 42.
The expression would be 3x - f = -6
All functions are relations, but not all relations are functions. A function is a relation that for each input, there is only one output.
EXPLANATION;
Functions are relations that have only one output for any unique input. To tell whether a graphed relation defines a function, you can use the vertical line test.
10x10=100
4.1x100=410
So, the answer is 410.