Simplify the expression by adding the polynomials:

(7y46y35y2+6y6)+(2y49y3+8y210y+9)\displaystyle {\left(-{7}{y}^{{4}}-{6}{y}^{{3}}-{5}{y}^{{2}}+{6}{y}-{6}\right)}+{\left(-{2}{y}^{{4}}-{9}{y}^{{3}}+{8}{y}^{{2}}-{10}{y}+{9}\right)}

The expression simplifies to