The solution of (3,13) means you need to create two lines that satisfy x = 3 and y = 13
The two lines can be anything as long as they give this solution
For example, if you wrote x + y = 16 (you know their values already)
Then write another y - x = 10
From here you have your equations, turn them into the f(x) form.
x + y = 16 can be turned into y = 16 - x which is f(x)=16-x
y-x = 10 can be turned into y = 10 + x which is g(x)=10 + x