Problem $2.$ Uniaxial Stress and Strain

An acetal polymer block is fixed to the rigid plates at its top and bottom surfaces. If the top plate displaces $2$ mm horizontally when it is subjected to a horizontal force $P=2kN,$ determine $($a$)$ the strain and $($b$)$ the shear modulus of the polymer. The depth $($dimension into page$)$ of the block is $100$ mm $.$ Assume that the polymer is linearly elastic and that the small angle approximation holds.