5а+7б-2а-8в

To simplify the expression \(5a + 7b - 2a - 8b\), you combine like terms.

1. Combine the \(a\) terms: \(5a - 2a = 3a\)
2. Combine the \(b\) terms: \(7b - 8b = -1b\) or simply \(-b\)

So, the simplified expression is:

\[ 3a - b \]