to do this we need to move 10 to other side. To accomplish this you just need to add 10 to both side since (-10)
so
A+ 10 = c -10 + 10
we get
A+ 10 = c
lets say it wasn't -10 but positive 10.
A = c + 10 then we would subtract 10 from both sides
A -10 = c + 10 - 10
we get
A - 10 = C
