5а+7б-2а-8в=

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

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

Putting it all together, the simplified expression is:

\[ 3a - b \]