The given expression is
5m+3m + 3n - m + 6n + 8
We would simplify the expression by collecting like terms.
The terms containing m are like terms
The terms containing n are like terms
8 is a constant
Thus, we have
5m + 3m - m + 3n + 6n + 8
7m + 9n + 8
The final expression is
7m + 9n + 8