Answer
$k=-12$
Work Step by Step
By the factor theorem, if $4x+3$ is a factor then $f(-\frac{3}{4})=0$. Thus $f(-\frac{3}{4})=20\cdot(-\frac{3}{4})^3+23(-\frac{3}{4})^2-10(-\frac{3}{4})+k=12+k=0$, so $k=-12$
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