To simplify the expression on the left-hand side, we add the like terms. This means we add numbers to numbers and combine the terms with z.
Combining the terms with z gives
[tex](2z+3z-7z)+(-8+10-7\text{)}[/tex][tex]-2z-5[/tex]
We see now that the term to be put on the blank on the blue line is -2z.
