Inverting (followed by multiplying) is what you do when you're dividing by a fraction. You're subtracting fractions. To add and subtract fractions (of like denominators), add and subtract their numerators.
By the way, if you are referring to question (3), "Simplify -M/7 - (1/7)(M - 9M)", the order of operations requires that you do the simplification inside the parentheses first, followed by the multiplication by the 1/7, only then moving on to combining with the -M/7.
3) "-9m" is not the correct simplification of the posted expression.
4) How did you get "45t"? What happened to your denominators? (It would really help if you started showing your work.)
5) Yes.
6) How do you get that "15" and "5" are the same number? How do you get that denominators "cancel out" when fractions are added? (Do you really believe that 1/2 + 1/2 equals 1 + 1 = 2, rather than 2/2 = 1?) Why are you not reducing fractions when possible? (Doesn't 15 go evenly into 60? Doesn't 5 go evenly into 120?) And where did "120/5" come from?
Eliz.