What value of x makes this sentence true? 5x+12=x-8

Get the variable on one side and the numbers on the opposite side. 

5x + 12 = x - 8

Let's get rid of that x on the right-hand side of the equation.

We can do this subtracting x from each side of the equation.

(5x - x) + 12 = (x - x) - 8 =
4x + 12 = -8

Let's take out the 12 so that the 4x can be alone.
We can do that by subtracting 12 from each side

4x + (12 - 12) = -8 - 12 =
4x = -20

And finally, divide the both sides by the coefficient of the 4x term. Which is 4.

4x / 4 = -20 / 4 =
x = -5

So, x is equal to -5.

So subtract x from both sides to isolate and get x on one side but you have to keep it positive inorder to make it easier so you get 4x+12=-8 then you subtract 12 from both sides to get x itself then you get 4x=-20 finally you divide by 4 and get that x=-5

The answer to your question is b