Block dropped on a spring

A 2.8 kg block is dropped from a height of 5.8 m (above the top of the spring) onto a spring of spring constant 3955 N/m. Find the speed of the block when the compression of the spring is 15.0 cm.

x=0.15m
k=3955N/m
m=2.8kg

Einitial mech=Efinal mech

Usi+Ugi+Ki=Usf+Ugf+Kf

Usi=0 because the spring is uncompressed
Ugi=???
Ki=.5mvi2 where $$V_i =sqrt(2g*5.8m)=10.868$$

Usf=.5kx2
Ugf=0 because I can set the height at that point to be 0
Kf=.5mvf2 where we're solving for v

How do I calculate the potential graviational energy? I'm guessing I need to find out how far the block would go if it were allowed to come to a rest, but I get an impossible to solve equation, or maybe I just don't remember what to do.

Block at rest:
Usi+Ugi+Ki=Usf+Ugf+Kf

0+mgx + .5mv2i=.5kx2
x(mg-.5kx)=-.5mvi2
x(27.468-1977.5x)=-165.359
x=???

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