A small object with mass mm $=0\mathrm{.}0900$ kgkg moves along the $+$x$+$x$-$axis. The only force on the object is a conservative force that has the potential$-$energy function U$($x$)=$x$2+$x$3$U$($x$)=$x$2+$x$3,$ where $=9\mathrm{.}00$ J$/$m$2$J$/$m$2$ and $=0\mathrm{.}300$ J$/$m$3$J$/$m$3.$ The object is released from rest at small xx$.$

Part A

When the object is at xx $=4\mathrm{.}00$ mm$,$ what is its speed?

Express your answer with the appropriate units.

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Part B

When the object is at xx $=4\mathrm{.}00$ mm$,$ what is the magnitude of its acceleration?

Express your answer with the appropriate units.