3a+7b -1,5a - 2,8b

To simplify the expression 3a + 7b - 1,5a - 2,8b, we can combine like terms.

First, let's combine the "a" terms: 3a - 1,5a = 1,5a.

Next, let's combine the "b" terms: 7b - 2,8b = 4,2b.

Finally, we can put the combined terms together to simplify the expression: 1,5a + 4,2b - 2.

So, the simplified expression is 1,5a + 4,2b - 2.