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x  is directly proportional to  y³     so     $x=ky^3$     where  k  is a constant.

y  is inversely proportional to  √z     so     $y=\dfrac{s}{\sqrt{z}}$     where  s  is a constant.

$x=ky^3\\~\\ x=k\Big(\frac{s}{\sqrt{z}}\Big)^3 \\~\\ x=\frac{ks^3}{z\sqrt{z}}$

When   z = 12 ,   x = 3     So...

$3=\frac{ks^3}{12\sqrt{12}}\\~\\ ks^3= 3\cdot12\sqrt{12}\\~\\ks^3=72\sqrt3$

When   z = 75 ,

$x=\frac{ks^3}{z\sqrt{z}}\\~\\ x=\frac{72\sqrt3}{75\sqrt{75}}\\~\\ x=\frac{72\sqrt3}{375\sqrt{3}}\\~\\ x=\frac{24}{125}$

X = k y^3       y = k/sqrtz      combine to

x = k /sqrt z ^3

or

xsqrt z^3 = k   always

3(sqrt12)^3 = xsqrt( 75) ^3

x = 3 (sqrt12)^3 /(sqrt75)^3

x = 3 *8 /125 = 24/125