Answer
$x=\pm6$
Work Step by Step
The distance formula from $P_1(x_1,y_1)$ to $P_2(x_2,y_2)$ is $d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}$.
Hence here: $\sqrt{(0-x)^2+(3-5)^2}=2\sqrt{10}\\x^2+4=40\\x^2=36\\x=\pm6$
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