1/3 - 1/12= 1/x

$$\frac{1}{3} - \frac{1}{12} = \frac{1}{x}$$

$$\frac{4}{12} - \frac{1}{12} = \frac{1}{x}$$

$$\frac{3}{12} = \frac{1}{x}$$  

Now by cross multiplying:

$$3x = 12$$

$$x = 4$$

x=4 because its to easy to solve, and thats the answer