I think the first thing you have to know that y = f(x). So for the given ordered pairs: (a, b) in the diagram, x = a and y = f(x) = b.

From the graph off, you can see that for any value ofxstrictly between 1 and 4, the value off(x) is greater than 0.

You can see from the diagram that the line of the graph is above the x-axis, and therefore positive, when x is between (1,0) and (4,0).

For any value ofxgreater than 8 and less than or equal to 10, the value off(x) is greater than 0.

You can also see the same from the diagram that the line of the graph is positive, when x is between (8,0) and (10,t). Here it doesn't matter what t is, all you need to know is that the line of the graph is above the x-axis.

For any other value ofxbetween -2 and 10 inclusive, the value off(x) is less than or equal to 0.

All other values of x not included in the above stated intervals, (between (1,0) & (4,0) and (8,0) & (10, t)), the graph is negative, as you can see in the diagram because the line of the graph is below the x-axis.

So the values ofxfor whichf(x) is positive are those for which 1 <x< 4 or 8 <x10.

Taking the text that I used to explain earlier, translate it into the language of math:

1. x is positive between (1,0) & (4,0): 1 < x < 4

2. x is positive between (8,0) & (10, t): 8 < x 10

*note: the difference between and is that the point (10,t) is not on the x-axis, so the point itself is included. (i.e. y = f(x) = f(10) = t)