Michael J. answered 02/04/15
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A function ƒ(x) is continuous if the limit of ƒ approaching from BOTH directions exist and if ƒ(x) is a value within the domains. So you must find where the limit does not exist from one direction and if there is no value within the domain.
Therefore:
ƒ(x)=x+1 is discontinuous at (-∞,2)
ƒ(x)=3x is discontinuous at (-∞,-1)∪(1,∞)
ƒ(x)=2x-1 is discontinuous at (-1,∞)