For which value(s) of a does the given system have infinitely many solutions?

{9x+y=5ax+9y=45\displaystyle {\left\lbrace\begin{array}{c} {9}{x}+{y}=-{5}\\{a}{x}+{9}{y}=-{45}\end{array}\right.} 

 a=\displaystyle {a}=