Question

What is the answer to this

Answer 1

The number that belongs in that green box is -1 because you are subtracting 1 from both sides in order to eventually isolate x.
Then you’ll have
-3x = -36 [divide by -3 to isolate x]
x = -36/-3
x = 12

Answer 2

When you subtract from a negative its more like adding - but keeping the negative sign.

so 35+1 
36

keep the negative!
-36 :)