Answer
$K = 0.75~J$
Work Step by Step
We can find $K$:
$K = mgy-\frac{1}{2}ky^2$
$K = mg(- \Delta y)-\frac{1}{2}ky^2$
$K = (20~N)(0.050~m)-\frac{1}{2}(200~N/m)(-0.050~m)^2$
$K = 1.0~J-0.25~J$
$K = 0.75~J$
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