Chapter 1.1, Problem 3QCE

State what must be true about $\underset{x\to a^{-}}{\lim }f\left(x\right)\text{ and }\underset{x\to a^{+}}{\lim }f\left(x\right)$ in order for it to be the case that $\underset{x\to a}{\lim }f\left(x\right)=L$