Solve the initial value problem below.

x2y7xy+16y=0,y(1)=4,y(1)=21\displaystyle {x}^{{2}}{y}{''}-{7}{x}{y}'+{16}{y}={0},\quad{y}{\left({1}\right)}=-{4},\quad{y}'{\left({1}\right)}=-{21}

y\displaystyle {y} =