$3($a$)$ A rectangular plate $30mm40mm$ is acted upon by a stress distribution which results in uniform strain $c_x=0\mathrm{.}0035,{}_y=0\mathrm{.}005,e_z=0,y_{xy}=0\mathrm{.}001875$ and ${}_{}$yzy=yzx=0, Determine the change in the length of the diagonals