Solve

y+8y+16y=0,y(0)=1,y(0)=9\displaystyle {y}{''}+{8}{y}'+{16}{y}={0},\quad{y}{\left({0}\right)}=-{1},\quad{y}'{\left({0}\right)}={9}

At what time does the function y(t)\displaystyle {y}{\left({t}\right)} reach a maximum?

t\displaystyle {t} =