In the graph you can see that f(x) and g(x) are lines, so the first step to know what function is h(x) is to find the functions f(x) and g(x).
Since a single line passes through two points, then to find the functions f(x) and g(x) you can take two points that pass through the graph of each function and obtain the slope, then use the point-slope formula. Then, you have
For f(x)
You can take for example the points (0,-3) and (3,0). Now calculating the slope:
Now using the point-slope formula:
Then, the function f(x) would be
For g(x)
You can take for example the points (0,4) and (4,0). Now calculating the slope:
Now using the point-slope formula:
Finally, the function h(x) will be