You can solve this by isolating the variable.
1.
30 + a = 40
You can isolate a by subtracting 30 from each side
30 + a - 30 = 40 - 30
a = 10
2.
500 + b = 200
Subtract 500 from each side
500 + b - 500 = 200 - 500
b = -300
3.
-1 + c = -2
Add 1 to each side
-1 + c + 1 = -2 + 1
c = -1
4.
d + 3567 = 0
Subtract 3567 from each side
d + 3567 - 3567 = -3567
d = -3567