Start finding the shorter diagonal $AC$
If we have $A(3,-4)$ and $C(1,2)$ then $AC=\sqrt{40}=2\sqrt{10}$
Call $O$ the midpoint of $AC$ and $B$ and $D$ the other vertices. From Pythagoras theorem $OB=\sqrt{BC^2-OC^2}=\sqrt{50-10}=\sqrt{40}=2\sqrt{10}$
The larger diagonal is $BD=4\sqrt{10}$ so the area of the rhombus is $\mathcal{A}=40$ and the height of the rhombus is $h=\dfrac{\mathcal{A}}{BC}=\dfrac{40}{5\sqrt{2}}=4\sqrt{2}$
$h=4\sqrt 2$
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