Use symbolic computations in Matlab to solve the initial value problem

$\displaystyle {y}'+{3}{y}={e}^{{-{3}{x}}}{\left({1}-{4}{x}+{3}{x}^{{2}}-{4}{x}^{{3}}\right)},\quad{y}{\left({0}\right)}=-{3}$

Note: use simplify$\displaystyle {\left({y}{\left({x}\right)}\right)}$ to obtain a simplified version of your solution $\displaystyle {y}{\left({x}\right)}$ obtained by the Matlab dsolve built-in function.