(2)8a+2b+5=2005

Get 4a+b =1000;

The algebraic expression =-12a+3b+5011.

If you look at it this way, there should be a sign that is wrong. If a coefficient is negative, you can substitute the result of the first step into the second step to get a specific value. If not, it can only be written as an expression of a or b.