2/3x=−1/3x−4 (they are not being divided)

2/3(x)-(-1/3(x)-4)=?
?=0

1÷3 = 0.3

2/3(x) - 0 -1/3(x) = ?
? = 0

2 ÷ 3 =.67

4(3)/ 3

your "x" becomes a negative

So, (-x) - 12 / 3 - 2/3(x)
we get the "12" from multiplying 4(3) from each other

Your like term of this problem▶ -x + 12

2(x) - (-x) - 12 / 3

3(x) + 12

x + 4 = 0
-4 -4

4x - 4

4x - 4 = 0

We have one solution to this equation

So, therefore your answer for this question is:
x = -4

5 gallons is your answer