$x-axis$ is perpendicular to plane inwards
$\frac{\mathrm{dx}}{\mathrm{dt}}=\frac{\mathrm{dy}}{\mathrm{dt}}=\mathrm{c} \quad \therefore \quad \frac{\mathrm{d}^{2} \mathrm{x}}{\mathrm{dt}^{2}}=\frac{\mathrm{d}^{2} \mathrm{y}}{\mathrm{dt}^{2}}=0$
Further $z=a x^{3}+b y^{2}$
$\therefore \quad \frac{d z}{d t}=3 a x^{2} \frac{d x}{d t}+2 b y \frac{d y}{d t}$
$=3 a c x^{2}+2 b c y\left[\frac{d x}{d t}=c=\frac{d y}{d t}\right]$
$\therefore \frac{\mathrm{d}^{2} z}{\mathrm{dt}^{2}}=6 \mathrm{acx}\left[\frac{\mathrm{dx}}{\mathrm{dt}}\right]+2 \mathrm{bc}\left[\frac{\mathrm{dy}}{\mathrm{dt}}\right]=6 \mathrm{ac}^{2} \mathrm{x}+2 \mathrm{bc}^{2}$
Now acceleration of particle is
$\overrightarrow{\mathrm{a}}=\frac{\mathrm{d}^{2} \mathrm{x}}{\mathrm{dt}^{2}} \hat{\mathrm{i}}+\frac{\mathrm{d}^{2} \mathrm{y}}{\mathrm{dt}^{2}} \hat{\mathrm{j}}+\frac{\mathrm{d}^{2} \mathrm{z}}{\mathrm{dt}^{2}} \hat{\mathrm{k}}=\left(6 \mathrm{ac}^{2} \mathrm{x}+2 \mathrm{bc}^{2}\right) \hat{\mathrm{k}}$
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