Obviously others have provided the answer, but what you need to understand is how to solve an equation. It’s an equation because both sides are equal, right?
So what you do to one side of the equation, you must do to the other side of the equation to keep it “equal.”
So to simplify the equation, move the whole numbers to one side and the unknowns (x’s) to the other.
You can move the 3 or the 13 first. It doesn’t matter which one, just pick one. To remove the 13 from the right side, subtract it. Since both sides must remain equal, you must subtract 13 from the left side too.
That leaves:
3 - 4x - 13 = 6x + 13 - 13
which is:
-10 - 4x = 6x
Now move the 4x from the left side to the right side. To do so, add 4x to both sides:
-10 - 4x + 4x = 6x + 4x
which is:
-10 = 10x
So 10 times what equals -10?
x = -1
(Last step is dividing both sides by 10 to move the 10 from the right side to the left side.)