For 12. Neil's mistake was that he didn't combine like terms of both the constant and numbers with both the variable of k. 9 and -8 are both constants so you combine it and get an answer of 1 since 9+ -8 is 1. 12k and k both have the variable of k so you combine it and get 13k.
The simplified version should be: 13k+1=12k-1
For 13. The third expression would be equivalent to the expression above listed as 12r+6s. The third expression is 4(3r+s)+2s and if you do PEMDAS you will get the same answer. Parentheses, 4 times 3r is 12r and 4 times s is 4s and after that you get 12r+4s+2s. As said before, if they have like terms you combine it so you get 12r+6s and it will equal the same as the one shown above.
Hope this helped you and would love brainiest! <span />
