The nonlinear spring has a tensile force$-$deflection relationship given by $F_s=200x+450x^2,$ where $x$ is in meters and $F_s$ is in newtons. Determine the acceleration of the $7-kg$ block if it is released from rest at:

$($a$)x=100mm,$ and

$($b$)x=200mm$

$($a$)a=\frac{m}{se{c}^2}$

$($b$)a=\frac{m}{se{c}^2}$