Continuity at a point and continuity of $x^2$ at $2$

Let \(a\in\mathbb R\) and let \(I\) be an open interval containing \(a\). For a function \(f:I\to\mathbb R\), give the definition of continuity at \(a\). Prove that \[ f(x)=x^2 \] is continuous at \(a=2\).