How do I solve 7b+5-7b-2=-8

A 'solution' to the equation is the value of  'b'  that makes the equation
a true statement.  Let's work on the equation and find it:

7b + 5 - 7b - 2 = -8
 
Notice that on the left side, you have  7b  and -7b .
When you combine those, they make zero-b, so you can get rid of them totally.

Now the equation says     + 5 - 2 = -8

Look at the left side again.  +5 and -2 combine to make  3 .

Now the equation says        3 = -8

Is there some value of  'b'  that can make this a true statement ?
I don't think so.
There is no solution to the original equation.
That happens, and it's OK.