Answer
$x^2+\dfrac{3}{2}y^2+2z^2$
Work Step by Step
Given: $f(x,y,z)=x^2+g(y,z)$
Also, $\nabla f =F$
Here, $g_y(y,z)=3y $
and $g(y,z)=\dfrac{3}{2}y^2+h(z) $ and $h(z)=2z^2+C$
Then, $f(x,y,z)=x^2+\dfrac{3}{2}y^2+h(z)$
Thus, $f(x,y,z)=x^2+\dfrac{3}{2}y^2+2z^2$