Question

What can be added to -1 so it can be a positive 1?

Answer 1

So represent missing number with x
x means unknown so
x+-1=+1
adding a negative means subtraction
x-1=1
you can do anything to an equation as long as you do it to both sides
add 1 to both sides
x+1-1=1+1
x+0=2
x=2

the number is 2