$$\frac{1}{3} - \frac{1}{12} = \frac{1}{x}$$
$$\frac{4}{12} - \frac{1}{12} = \frac{1}{x}$$
$$\frac{3}{12} = \frac{1}{x}$$
Now by cross multiplying:
$$3x = 12$$
$$x = 4$$
.$$\frac{1}{3} - \frac{1}{12} = \frac{1}{x}$$
$$\frac{4}{12} - \frac{1}{12} = \frac{1}{x}$$
$$\frac{3}{12} = \frac{1}{x}$$
Now by cross multiplying:
$$3x = 12$$
$$x = 4$$