The domain can be determined by observing what x-values are are covered by the graph (or what x-values have points on the graph).
Work Step by Step
When given the graph of a function, its domain can determiend by observing what x-values have points on the graph. $\\$For example, if the graph is a parabola, then the domain will be the set of real numbers as the parabola covers all the x-values horizontally. $\\\\$On the other hand, if the x values covered are only from 0 and above,m then the domain will be $[0, +\infty)$.