The dashed vertical line is a vertical asymptote. The x-coordinate of all points on this vertical asymptote are equal to 7. In other words, when the x-value of a point on the graph approaches , its y-value approaches infinity (or negative infinity.)
Either way, the graph is not defined for . The point should thus be excluded from the domain of the graph.
The graph is apparently defined for all other x-values. The domain of the function should thus be all real numbers with the exception of . Here's how to write that in interval notation:
The set of all real numbers can be expressed as the interval . Note that infinity (or itself isn't a real number. The round brackets indicate that both endpoints are excluded from the from the interval.
The set of all real numbers less than (not equal to) is (both ends are excluded.) The set of all real numbers greater than (not equal to) is .
The set of all real numbers that is not equal to is the union of all real numbers less than and all real numbers greater than . The union operator connects two intervals.