Use Gauss's law: $\oint \mathbf{E}\cdot d\mathbf{A}=Q_{\rm enc}/\varepsilon_0$. For a spherical Gaussian surface of radius $r$ this gives $E(r)\,4\pi r^2=Q_{\rm enc}/\varepsilon_0$, so
$E(r)=\dfrac{Q_{\rm enc}}{4\pi\varepsilon_0\,r^2}$. The magnitude is
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