Answer:
x=9
Explanation:
Since both expressions are formed from two parallel line with a bisector, you know they would be equal to each other.
We first start with the equation
13x+3=12x+12
Now we want x only on one side, so we subtract 12x from both sides
13x-12x+3=12x-12x+12
Now we simplify
x+3=12
Now we want to isolate x, so we subtract 3.
x+3-3=12-3
Finally we simplify
x=9