how do i define domain and range from looking at the graph?

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Basically, you can find the dom/ran from a graph by looking where the line stops dead. Like when y = √x, the line stops dead where x = 0 and downwards, so x must be more than or equal to 0.There's also another one too... If you see a parabola (a straight line where the graph will never touch but gets increasingly closer to it) its eithere going to be an inequality domain and range (e.g. y=(1/x^2) has a parabola of y=0, so  y will have a range greater than 0), or a "not equal to" (e.g. y = tanθ has a parabola at 180 degrees, so y is not equal to 180)