1. 9m = 81 m = 2. 75 = 3n n = 3. r ÷ 7 = 12 r =

1.9m=81  divide each side by 9 so we leave m alone, which cancels out the 9 on the left side leaving m=81/9  which simplifies to m=8

2.75=3n again we need to get n alone, so we are going to divide by 3 on each side.  This cancels the 3 on the right side, leaving 75/3=n  simplified to 25=n or n=25

3. r÷7=12  We need r alone.  We always do the opposite of what is already being done.  So in this case 7 is being divided.  The opposite of divide is multiply (or times).  So we are going to multiply each side by 7.  This will cancel the 7 out on the left side and leave r=12x7.  So r=84

So in the first 2, they were being multiplied, so we did the opposite and divided.  
In the last one, it was being divided, so we did the opposite and multiplied.
If it was being added, we would subtract.
If it was being subtracted, we would add.

Hope that helps!  Good luck!