12. It's similar to having an infinite number of solutions because there isn't a specific solution to answer the question.
13. For the equation 0.9x + 5.1x - 7 = 2(2.5x-3), there is only one solution and it is x=1
14. The error that your friend made was that she added extra 16x to the equation. the correct answer is that there is no solution.
16. the equation 6 (x+2)= 5 (x+7) has only one solution. it is x=3.
sorry i wasn't able to answer the other 3,
by the way you don't need to send money, because we are here to help