For the first question, subtraction and addition do have a relationship. Namely, they have an inverse relationship to each other. Subtraction is inverse to addition and vice versa.
For the second question, you can draw a number then plot the numbers. b is subtracting from a. So, move the point a back b times.
And for the last question, you can find out what sign the answer will have by looking at a and b. If the number you are subtracting from is less than the number doing the subtracting then the answer will have a negative sign.