Notice that the line segment that passes through the point (0,0) has a slope of 2, while the line segment that passes through the point (5,3) has a slope of 1.
From the given linear equations, the function y=2x is the only one which has a slope of 2.
To find the domain in which the equation y=2x is used to define f(x), look at the values over the x-axis where the line segment lies.
Notice that the line segment with slope 2 ends at the value x=2 and it seems to keep going all the way to -∞. Then, the equation y=2x is used to define f(x) on the following part of the domain:
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