Acceleration from distance?

Acceleration $a(t)$ is given below:

$\begin{aligned} a(t) & = \frac {dv(t)}{dt} \\ & = \frac {dv(t)}{dx} \cdot \color{#3D99F6}{\frac {dx}{dt}} \\ & = \color{#3D99F6}{v(t)} \frac {dv(t)}{dx} \\ & = \beta x^{2n} \frac d{dx} \beta x^{2n} \\ & = \beta x^{2n} \cdot 2n\beta x^{2n-1} \\ \implies a(t) & = 2n\beta^2 x^{4n-1} \\ a(t) \big|_{x=3} & = 2\cdot 1 \cdot 1^2 \cdot 3^{4-1} & \small \color{#3D99F6}{\text{for }n = 1, \ \beta =1} \\ & = \boxed{54} \end{aligned}$