Hello there.
First, we need to isolate x to one side of the equation. To do this, we need to cancel out one of the x's by adding or subtracting it's opposite. Since the right side of the equation has a lower value, we'll cancel that one.
Remember, what you do to one side of the equation, you must do to the other side.
3x + 8 = -x + 20
Add 1x to both sides (this cancels out -x and adds 1x to 3x).
4x + 8 = 20
Now we need to isolate the variable. Our current equation has the variable being added to a number, so we need to cancel out that number. To do this, we need to subtract the value of the number.
4x + 8 = 20
Subtract 8 from both sides.
4x = 12
Now, all we need to do is perform the opposite order of operations to 4x in order to solve for x. Since x is being multiplied by 4, we need to divide both sides by 4.
x = 3 is your answer.
I hope this helps, and have a great day!