Hey there!
First of all, we need to see which terms are like terms. Like terms are parts of the expression that can be added together. The thing is, you cannot add, for example, 4x and 5, in an expression because not knowing what x equals, we cannot simplify it anymore. But, we could add 4x and 3x, giving us 7x.
By definition, like terms contain the same variable and are raised to the same power. You can't combine 5x² and 5x in an expression for the same reason you can't add 4x and 5.
So, in our equation, we have 6b²,3b,-5b², and -8b. We can see two types of terms, the b and the b², so we will separate them accordingly.
6b²-5b²
and
3b-8b
Now, we simplify these.
b²
and
-5b
Now, we put them back together into one expression.
b²-5b
I hope this helps! Have a great day!