To solve this equation, we can use the cross multiplication method.

First, we multiply the denominator of the left side by the numerator of the right side and set it equal to the numerator of the left side multiplied by the denominator of the right side:

5 4 = (1/3x) 12

20 = 4x

Next, we divide both sides by 4 to solve for x:

20/4 = 4x/4

5 = x

Therefore, the solution to the equation is x = 5.