RE:
Actually the Range of the function
f(x)= x^2-3x
is
(-2.25,INF)
yes, but the domain of the function you graphed is (-inf, inf), so it is not the same function we are talking about. our function has the domain {-3, 1, 10} only, so these are the only possible x-values we can plug in. and so, the range is the set of values we get after plugging each of those values into our function.
remember, the domain is the set of inputs for which a function is defined, the range is the set of outputs for which a function is defined